A UFO or Unidentified Flying Object is simply defined as any object or optical phenomenon observed in the sky which cannot be identified, even after being thoroughly investigated by qualified people.

ex·tra·ter·res·tri·al (ĕk'strə-tə-rĕs'trē-əl) Originating, located, or occurring outside Earth or its atmosphere: intelligent extraterrestrial life.An extraterrestrial being or life form.

Ufology is the study of unidentified-flying-object (UFO) reports, sightings, and other related phenomena.