A UFO is something seen in the sky that the person watching cannot identify. That is all the letters actually say — unidentified flying object. But in real use the word has drifted a long way from that plain meaning: when people say UFO, most listeners hear 'alien spaceship', and that is usually what the speaker means too. This is why serious writers and government reports have moved to the newer term UAP, unidentified aerial phenomenon, precisely to get away from the spaceship feeling that UFO now carries. So the word sits in two places at once. Used carefully, it is honest and modest: I saw something, I do not know what it was. Used the ordinary way, it belongs to films, tabloid newspapers and late-night stories, and it carries a slight smile — call a person a UFO believer and you are half teasing them. It is a countable noun: one UFO, three UFOs. People say the letters one by one, 'U-F-O', though 'yoo-foh' as a single word exists and sounds more casual or joking.