Unidentified
Flying
Object
That is - something in the sky that you cannot identify.
Could be anything. aircraft, bird, balloon, cloud....if you look up into the sky, see something and think "oooh, I wonder what that is" you have seen a UFO.
UFO does NOT mean spaceship and people who think it does are very very very very very very very very very very STUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUPID