UFO, or Unidentified Flying Object, is a term used for anything that is seen in the sky that can not be identified. Yes, most of the time that would mean spaceships and or objects from other planets. Most often through it is usually an airplane, bird, weather phenomenon, etc. (Yes, I said most often, some things remain unexplained and you just never know.)
The term UFO dates back to 1953. Donald Keyhoe used it in a book about strange things he saw during his time in the service. In 1956, USAF office Edward Ruppelt claims to have coined the phrase to replace the more common term of the day. Flying Saucer. That term itself came into being in the 1940’s and was used to describe anything that could not be accounted for flying in the sky.