UFO is an abbreviation for Unidentified Flying Object. Although this could describe any object in the sky that cannot be identified by an observer, it usually is associated to vehicles from another planet piloted by aliens—entities from that planet. Although most such sightings of UFOs are eventually identified, the quantity of sightings by qualified, expert observers (such as professional pilots) or by large numbers of independent observers that remains unidentified is considerable.

Due to the massive distance between planets and current inability of people on earth to know of a way to traverse such distances quickly, some researchers have developed the belief that UFOs may be from the future, from another dimension, or are unusual—and unexplained—natural phenomena.