A UFO remains a mystery even after all attempts of identification have been made. Here's a more thorough definition of UFO by J. Allen Hynek, the astronomer who spent years studying the phenomenon:
"The reported perception of an object or light seen in the sky or upon the land the appearance, trajectory, and general dynamic and luminescent behavior of which do not suggest a logical, conventional explanation and which is not only mystifying to the original percipients but remains unidentified after close scrutiny of all available evidence by persons who are technically capable of making a common sense identification, if one is possible."