An unidentified flying object (UFO) is any perceived aerial phenomenon that cannot be immediately identified or explained. Upon investigation, most UFOs are identified as known objects or atmospheric phenomena, while a small number remain unexplained. Small but vocal groups of ufologists favour unconventional or pseudoscientific hypotheses, often claiming that UFOs are evidence of extraterrestrial intelligence, technologically advanced cryptids or interdimensional contact, but even after decades of promotion of such ideas by believers and in poplar media, the kind of evidence required to solidly support such claims has not been forthcoming.

Source: Wikipedia