An object seen, or apparently seen, in the sky and whose nature cannot be determined by the observer. By far the majority of sightings can be explained in terms of known phenomena with which the individual observer was unfamiliar (e.g. the planet Venus, high altitude balloons, artificial satellites, etc.), but there remains a residue of sightings which cannot adequately be explained, either because of an insufficiency of reliable data or, possibly, because a phenomenon which is not yet understood has genuinely been observed.
The most popular explanation (and certainly the most glamorous one) is that such unexplained sightings are due to extra-terrestrial spacecraft, or FLYING SAUCERS, to use a popular term. While the possibility of the Earth's being visited by alien spacecraft is one which cannot wholly be excluded, the nature and frequency of such sightings makes this "explanation" a most improbable one.
