An unidentified flying object (UFO) is an object or phenomenon seen in the sky but not yet identified or explained. The term was coined when United States Air Force (USAF) investigations into flying saucers found too broad a range of shapes reported to consider them all saucers or discs. UFOs are also known as unidentified aerial phenomena or unidentified anomalous phenomena (UAP). Upon investigation, most UFOs are identified as known objects or atmospheric phenomena, while a small number remain unexplained.

While unusual sightings in the sky have been reported since at least the 3rd century BC, UFOs became culturally prominent after World War II, escalating during the Space Age. Studies and investigations into UFO reports conducted by governments (such as Project Blue Book in the United States and Project Condign in the United Kingdom), as well as by organisations and individuals have occurred over the years without confirmation of the fantastical claims of small but vocal groups of ufologists who favour unconventional or pseudoscientific hypotheses, often claiming that UFOs are evidence of extraterrestrial intelligence, technologically advanced cryptids, demons, interdimensional contact or future time travelers. After decades of promotion of such ideas by believers and in popular media, the kind of evidence required to solidly support such claims has not been forthcoming. Scientists and skeptic organizations such as the Committee for Skeptical Inquiry have provided prosaic explanations for UFOs, namely that they are caused by natural phenomena, human technology, delusions, and hoaxes. Although certain beliefs surrounding UFOs have inspired parts of new religions, social scientists have identified the ongoing interest and storytelling surrounding UFOs as a modern example of folklore and mythology understandable with psychosocial explanations.

The problems of temporarily or permanently non-knowable anomalous phenomenon or perceived objects in flight is part of the philosophical subject epistemology.