Acronym formed from the phrase Unidentified Flying Object, a title created by the USAF for an object that is typically airborne and doesn't appear to be a known natural or manmade object or phenomenon, or is the subject of a UFO report. Originally pronounced yoo-foe as a short form in speech, today the word is usually sounded out using the name of each letter and is used to convey the idea of an alien craft.


The word UFO was created during 1952 to replace the phrase flying saucers 1, which at the time were popularly believed to be spacecraft from another world. The replacement was made to accommodate shapes and sizes other than disks and to distance the USAF from the public impression that they were investigating something of extraterrestrial origin. However this latter strategy was rather transparent and consequently the vast majority of written and graphical portrayals convey the idea that UFOs are some sort of alien craft.

In the lexicon of ufology the word UFO also has different meanings in different contexts. While it is clear that the word is meant to be applied to objects that appear to be extraordinary or out of this world, not all the objects in UFO reports turn out to be UFOs. Therefore for the purpose of investigation, the objects in UFO reports are not classed as UFOs until the reports have been investigated and all known natural or manmade objects have been ruled out with reasonable certainty. In his classic book The UFO Experience, eminent ufologist and astronomer Dr. J. Allen Hynek puts it this way:

We can define the UFO simply as the reported perception of an object or light seen in the sky or upon land the appearance, trajectory, and general dynamic and luminescent behavior of which do not suggest a logical, conventional explanation and which is not only mystifying to the original percipients but remains unidentified after close scrutiny of all available evidence by persons who are technically capable of making a common sense identification, if one is possible.

Many different shaped UFOs have been reported, but the most ubiquitous are spheres and disks. More recently, triangular craft have also been reported. Disk and triangular shaped UFOs often have a raised section in the center-top of the craft that is presumed to be a crew cabin. Most UFOs are fairly small, usually under 150 feet wide. However on rare occasions gigantic UFOs have also been sighted. On radar some UFOs also appear to merge, and this merging has led mainstream ufologists to speculate that large UFOs are carriers, also called mother ships, and that the smaller UFOs are probably reconnaissance craft or probes. The artist's rendition ( top-left ) depicts a saucer shaped UFO purportedly under study at a USAF test facility known as Area 51 in Nevada USA.


In his classic book The Report On Unidentified Flying Objects, United States Air Force Captain Edward J. Ruppelt takes credit for creating the term UFO to replace the words flying saucers.1 Technically, this makes it a euphemism for flying saucers, which were popularly presumed to be craft of alien origin. However the word UFO allows for various other configurations in addition to the classic disk shaped craft. While it seems true that Ruppelt introduced the acronym around the time he took over Project Grudge, documents dated February 1949 and most probably drafted during 1948 as part of Project Sign use the full phrasing "unidentified flying object". One such quote can be found in Technical Report No. F-7R-2274-IA Unidentified Aerial Objects Project "SIGN", written by Lawrence H. Truettner and Albert B. Deyarmond:

"No definite evidence is yet available to confirm or disprove the actual existence of unidentified flying objects as new and unknown types of aircraft. "2

Another instance from a routing and record sheet used for inter-office correspondence at Air Materiel Command dated February 13, 1948 reads:

"Referenced item states Kodachrome pictures were taken of an unidentified flying object by [ NAME WITHELD ] of Harmon Field, Newfoundland on 10 July, 1947. It is requested that the films or photographs be obtained for study."3

Ruppelt does not appear to have worked with Truettner or Deyarmond, but early in 1952, Ruppelt viewed microfilm archives from Project Sign, so he could have run across the phrase there. It was also Ruppelt who drafted the review of the UFO situation that was presented to General Samford in 1951. It is unlikely that any such review would have been submitted without taking into account the work done by Project Sign. Additionally, because some of the microfilm in the Project Sign archives had been damaged, Ruppelt took it upon himself to locate and interview some of the original Project Sign staff, including Al Deyarmond.4 These factors make it highly probable that Ruppelt did not hit upon the phrase independently. Rather it seems likely that while researching his review for Samford, he ran across several different terms that all referred to the same phenomenon, then gravitated toward the one that best fit his needs.

Ruppelt's superiors also wanted something that wouldn't be construed as an overt endorsement of the ETH, and the phrase Unidentified Flying Object served that purpose. Because it was Ruppelt who struck upon the idea of reducing it to acronym form and designating it for official use on USAF projects, it is fair to say that Ruppelt created the word UFO. One of the first documented instances of usage is in Project Blue Book Status Report No. 8 dated 31 December 1952. An incident involving the observation of a UFO reads as follows:

"At approximately 0949 MST on 29 July 1952, several pilots and guards from Los Almos observed an UFO. The object was flying straight and level at high speed north of the Los Almos landing field. The object, which was shiny metallic color was observed for 30 minutes with binoculars."5

Official USAF Definitions

Prior to the inception of the word UFO, the USAF often referred to flying saucers as unidentified aerial objects, and loosely defined them as any aerial object which the observer is unable to identify. In more serious cases the object took on greater significance and were classed as an Unknown. Being an Unknown meant that the sighting report came from a stable source, contained a reasonable amount of data, and that after exhaustive investigation, the object that was reported could not be identified6 .

Because the Unknowns were the ones the USAF was most interested in, procedures for screening sighting reports prior to being sent to ATIC were introduced. A first step in this direction was Air Force Letter 200-5 dated April 29, 1952, titled Unidentified Flying Objects Reporting ( Short Title: FLYOBRPT ). AFL 200-5 was instituted by Order of the Secretary of the Air Force and handed down through USAF Chief Of Staff Hoyt S. Vandenberg. It briefly outlined reporting procedures for base commanders and defined a UFO as, "any airborne object which by performance, aerodynamic characteristics, or unusual features does not conform to any presently known aircraft or missile type."

AFL 200-5 did not result in sufficient improvement to the quality of reports being submitted to Project Blue Book, so on July 25, 1953, ATIC released a 68 page guide titled How To Make FLYOBRPTS. It was stamped "Restricted" and included expanded interpretations plus a 9 page questionnaire for UFO witnesses ( civilian and military ). The guide defined UFOs in this way:

"It is impossible to define accurately the term "flying saucer" or unidentified flying object. For all practical purposes the term may be applied to any airborne object which by performance, aerodynamic characteristics, or unusual features fail to conform with presently known aircraft or missile types, and cannot be identified as a known object or phenomena ( i.e., known objects being defined as balloons, aircraft, etc. )."

The next evolution in defining UFOs came with Air Force Regulation 200-2. The version issued on August 12, 1954 by USAF Chief Of Staff, Nathan F. Twining refers to unidentified flying objects as UFOBs and Paragraph 2a defines them as, "any airborne object which by performance, aerodynamic characteristics, or unusual features does not conform to any presently known aircraft or missile type, or which can not be identified as a familiar object." It then goes on in paragraph 2b to loosely define familiar objects as a separate group: " Familiar Objects - Include balloons, astronomical bodies, birds, and so forth."

Paragraph 2b also had special significance when it came to the release of information to the public. It is referred to under paragraph 9, Release of Facts, which includes this statement: "In response to local inquiries, it is permissible to inform news media representatives on UFOB's when the object is positively identified as a familiar object (see paragraph 2b)" It also goes on to say, "For those objects which are not explainable, only the fact that ATIC will analyze the data is worthy of release, due to the many unknowns involved." At this point it became clear that only reports that could be explained as mundane objects or phenomena were to be released to the public, and the most interesting cases were to be withheld for further analysis by ATIC.

AFR 200-2 went unchanged until February 05, 1958. It was then revised again to make it very clear that the USAF did not merely consider UFOs to be unknown objects or aircraft, but that they are something else altogether, and were not to be confused with known types of objects, including unknown aircraft. Anything that indicated that the UFO could be a known object or some sort of unknown aircraft was not to be reported as a UFO. The relevant sections are reproduced below:

Excerpt From AFR 200-2, February 05, 1958

2. Definitions. To insure proper and uniform usage in UFO screenings, investigations, and reportings, the objects are defined as follows:

  1. Familiar or Known Objects - Aircraft, birds, balloons, kites, searchlights, and astronomical bodies (meteors, planets, stars).
  2. Unknown Aircraft:
  3. (1) Flying objects determined to be aircraft. These generally appear as a result of ADIZ violations and often prompt the UFO reports submitted by the general public. They are readily identifiable as, or known to be, aircraft, but their type, purpose, origin, and destination are unknown. Air Defense Command is responsible for reports of "unknown" aircraft and they should not be reported as UFO's under this regulation.
    (2) Aircraft flares, jet exhausts, condensation trails, blinking or steady lights observed at night, lights circling or near airports and airways, and other similar phenomena resulting from, or indications of aircraft. These should not be reported under this regulation as they do not fall within the definition of a UFO.
    (3) Pilotless aircraft and missiles.
  4. Unidentified Flying Objects - Any airborne object which, by performance, aerodynamic characteristics, or unusual features, does not conform to known aircraft or missiles, or which does not correspond to definitions in a. and b. above.
Defining Unknown & Unidentified

Official USAF definitions clearly show that for a sighting report to be considered an "Unknown" there must be enough data to exclude with reasonable certainty any known manmade or natural phenomena, including unknown aircraft or any other thing with characteristics that merely suggest aircraft as an explanation. Yet some skeptics continue to argue that because the acronym UFO includes the word "Unidentified", UFOs could be anything at all, including those things that the official definition excludes. Skeptics also suggest that if sufficient data was available, all UFOs would turn out to be natural or manmade objects or phenomena. To correct these misconceptions, consider the following USAF quote on the meaning of the word "unidentified" in the context of UFO investigations:

"A sighting is considered unidentified when a report apparently contains all pertinent data necessary to suggest a valid hypothesis concerning the cause or explanation of the report but the description of the object or its motion cannot be correlated with any known object or phenomena."7

So what becomes increasingly evident is that as investigative protocols evolved, the word "UFO" was not meant to be interpreted merely as the literal definition of the words that form the acronym, but as something which is observed well enough to determine that it doesn't correspond to any known object or phenomenon. To emphasize this differentiation further, reports of such objects were labeled "Unknown" and catalogued separately from sightings with insufficient data ( see chart below ).

Sighting Evaluations

The chart on the left spans about five years and takes into account 1,593 of the best cases from 1947-1952. By best cases, we mean that for a case to be included in the statistics, it had to pass an initial screening to determine if closer study was warranted. This was typical procedure for all Project Blue Book reports. To quote Edward Ruppelt,
"Since June 1947, when the first UFO report had been made, ATIC had analyzed 1,593 UFO reports. About 4,400 had actually been received, but all except 1,593 had been immediately rejected for analysis."
The screening protocols indicate a high level of objectivity among USAF investigators. Indeed, to some extent it represents an anti-ufo bias, because if they had wanted to create the impression of a more critical situation, it would have been more effective to use every UFO report available. Add to the process that even after initial screening, over 20 % of the reports proved too vague to be included in any category other than Insufficient Data. Nevertheless, analysis by both military and civilian experts still pegged the amount of Unknowns at nearly 27%.

The chart above also takes into account hoaxes and manmade or natural phenomenon all the way down to bits of blowing paper.8 Lastly, a layer of skeptical bias is evidenced by the placement of uncertain reports into categories other than Unknown based on the mere probability of them being identified if more information had been available. This clearly nullifies the claim that if more information were available then all UFOs would be explained as mundane objects.

Sighting reports came from pilots, air crews, scientists, engineers, radar operators, military and civilian observers in general. The UFOs they described were often spherical or disk shaped and could perform maneuvers impossible for any terrestrial aircraft then or now. Perhaps some twenty-first century technology might yet be invented that can duplicate the performance characteristics of UFOs, but certainly there was nothing that came close back in the 1950s. Even the legendary YF-12A and X-15, regardless of their phenomenal speed, were incapable of making instantaneous high-speed turns.

Add to the above another half-century of sightings by thousands of people who have become increasingly better informed due to advances in education and the growth of the aircraft industry, and we are left with only one reasonable conclusion. To borrow a quote created by Sir Arthur Conan Doyle, "We must fall back upon the old axiom that when all other contingencies fail, whatever remains, however improbable, must be the truth." If no manmade or natural object or phenomenon can reasonably explain all UFO sightings, then some UFO sightings must represent a form of alien technology ( ATECH ). The initial findings of Project Sign are therefore probably correct and we are probably dealing with something extraterrestrial.

Although the ETH is the most probable explanation for UFOs, not much else has been established, or if it has, the public is not aware of it. Virtually everything else we know about UFOs is based on observation, extrapolation and speculation. We don't know where UFOs originate, but it is highly doubtful they are from any planet in our solar system. We don't know why they are here, but curiosity about any inhabited planet would be understandable. We don't know how UFO technology works, but gravitational propulsion would explain their performance.

Claims by contactees or abductees cannot be relied upon, not necessarily because the abduction phenomenon doesn't exist, or that alien contact hasn't taken place, but because no information provided by any contactee or abductee has ever revealed anything beyond the scope of the technology or scientific understanding of the day. Worse yet, the stories of contactees and abductees are more often than not filled with trite commentary that contradicts scientific fact and/or has been shown to be based on fiction or fabrication. That is not to say that all those who claim to have had alien contact are hoaxers. It just means that genuine experiencers haven't been given any unique and verifiable information. Lastly, there is little doubt that military establishments of the world know more about UFOs than has been disclosed to general public. However it is also doubtful that even the military has all the answers.

The Semantics Problem
I cannot think of anything more treacherous than this label "unidentified". - Jacques F. Vallée

Astronomer and ufologist Jacques Vallée was among the first to tackle the semantics problem. The quote above is from his classic book Anatomy of a Phenomenon. It speaks to a problem that has been a thorn in the side of ufologists since the word UFO was invented. Although the word "UFO" and the phrase "flying saucer" are now firmly imbedded in the modern English language, and are synonyms used to convey the idea of alien craft, the use of the word "Unidentified" has left the door open for skeptics to claim that UFOs could be anything at all, including all those things that the official definitions above were meant to exclude.

To avoid the semantics problem, other terminology has been proposed. For example, the blogger Kyle King proposed Near Earth Object ( NEO ), which has a nice ring, but includes too broad a range of subject matter. When referring to sightings that may include a wide range of aerial anomalies including those of natural origin, USI has adopted the term UAP ( unidentified aerial phenomena ), as introduced by Dr. Richard F. Haines around 1980 and used frequently by NARCAP. But again, UAP is still too non-specific when what we are really talking about are alien craft.

Closing the gap somewhat is the acronym ATECH. Introduced by the Ufology Society International (USI) during 1995, it stands for "Alien Technology" and includes alien transportation (ATRANS), alien communications (ACOM) and any other kind of alien technology that may be applicable. While ATECH or ATRANS are better choices for replacing the term UFO, it still doesn't capture the essence of what we are getting at. A transit system on a distant planet or a radio signal from a distant star might confirm that ATECH exists elsewhere in the universe, but it’s not the same as seeing a UFO off your starboard wing.

In the past, USI has also suggested the acronyms ETV ( Extraterrestrial Vehicle ) and AT ( Alien Transport ), but neither caught on, probably because it's easier to simply say "alien transport" or "extraterrestrial vehicle" than it is to explain the acronyms. Besides that, because they end up meaning the same thing as UFO anyway, using them becomes redundant. This brings us to the issue of the word "alien", which has both terrestrial and extraterrestrial connotations. What we really mean is alien to human civilization or extraterrestrial, but not necessarily or simply from space. Meteors are from space, but they aren't intelligently controlled. Space shuttles are intelligently controlled, but we aren't talking about them either.

A Contemporary Definition

The semantics problem poses a serious challenge for those who want a well formed acronym that will gain acceptance. To address this issue USI proposed in 2010 that we simply retain the word UFO and define it to reflect what the word is meant to convey in modern language rather than continuing to use historical definitions that define UFOs as what they aren't. For example, historical definitions, particularly the official USAF versions seen above, take the approach of eliminating as many known natural and manmade objects and phenomena as possible and then define UFOs as whatever is leftover. Inevitably that approach implies something alien without having to state it directly. That was very convenient for officials who didn't want to use language that reinforced the idea that UFOs are alien craft, but the general public knows what the heart of the UFO issue is really all about and isn't obliged by political correctness to mince words. Therefore it isn't surprising that the word UFO has evolved to become the most common word used in contemporary English Language to convey the idea of alien craft.

To resolve the conflict that arises when the word UFO isn't meant to convey the certainty of an alien craft, such as in the context of a UFO report, we can add a second level interpretation to be used in the context of UFO reports or investigations. Lastly, we can resolve the issue of literal misinterpretations by keeping the acronym separated from the definition by way of the word origin. The resulting definition for UFO is as follows:

UFO or ufo

Pronunciation: yoo-ef-oh ( plural UFOs ) or yoo-foe ( plural ufos ) noun
  1. A craft of alien origin.
  2. The object or phenomenon that is the focus of a UFO report or investigation.
Word Origin: [ Mid-20th century (1952) acronym formed from the words unidentified flying object. ]9

NOTE: This definition may be freely reproduced by dictionary publishers without restriction in any format.


  1. The Report On Unidentified Flying Objects | eBook Page 1
  2. NARA Blue Book Roll 85 | NARA-PBB85-6
  3. NARA Blue Book Roll 02 | NARA-PBB2-36
  4. The Report On Unidentified Flying Objects | eBook Page 38
  5. NARA Roll 30,362 | USAF Blue Book Status Report No. 8, Dec. 31, 1952 | MAXW-PBB1-676
  6. The Report On Unidentified Flying Objects | eBook Page 10
  7. NARA Blue Book Roll 87 | NARA-PBB87-45
  8. The Report On Unidentified Flying Objects | eBook Page 216


  1. The UFO in the artist's rendition ( top left ) depicts an alien transport purported to have been under study at Area 51. According to author Robert Lazar, who claims to have worked at the Area 51 S-4 facility where the craft was being studied, it is 52 feet in diameter and christened the "sport model". Its actual existence has not been verified and it is deemed likely to be a hoax by skeptics and conservative ufologists.
  2. The Sighting Evaluation Chart was constructed by the article's author from data in Captain Edward J. Ruppelt's book The Report On Unidentified Flying Objects ( eBook Page 216 )