Unidentified flying object

A UFO (Unidentified Flying Object) is any object flying in the sky which cannot be identified by the person who sees it. Sometimes the object is investigated. If people can still not figure out what the object is after an investigation, it is called a UFO. If they figure out what the object is, it can no longer be called a UFO because it has been identified.

Even though UFOs might have different origins, people often use the word UFO when they are talking about alien spacecraft. Flying saucer is another word that is often used to describe an unidentified flying object. Some UFOs that are seen in the water, or going in or out of the water are called USOs (Unidentified Submerged Objects).

Identified flying objects (IFOs)

Studies estimate that 50-90% of all reported UFO sightings are identified later. Usually 10-20% are never identified. Studies also show that very few UFO sightings are hoaxes (people trying to trick other people). Most UFOs are actually natural or man-made objects that looked strange.

80-90% of IFOs are identified as one of three different things:

astronomical causes (for example: planets, stars, or meteors)
aircraft
balloons (including weather balloons)

10-20% of IFOs are other causes (such as birds, clouds, mirages, searchlights, etc.)

References

Allan Hendry, The UFO Handbook: A Guide to Investigating, Evaluating, and Reporting UFO Sightings, 1979, Doubleday & Co., ISBN 0-385-14348-6

Other websites

Wikinews - UFO Archive
CIA educational summary on UFO Archived 2007-05-09 at the Wayback Machine (working link or what? Maybe it should be used this link Archived 2019-10-01 at the Wayback Machine instead?)
NASA Institute for Advanced Concepts
BBC article on Mexican Air-force videotape
NASA UFO Hacker Video Interview Archived 2007-11-13 at the Wayback Machine
Popularmechanics Article - When UFOs Arrive: February 2004 Cover Story Archived 2007-09-27 at the Wayback Machine