That's not the definition. Are you saying than any unidentified object ever seen is a flying saucer?
You're right, it's not the definition. But to a lot of people (I don't know the stats here but based on my experience) "U.F.O" is not an acronym where one examines each of the 3 letters seperately and literally, but instead, is a word used to connote an object in the sky that invokes highly advanced technology beyond terrestrial explanation. I suppose, we could even say that to many people, the "U" in "UFO" means "unexplainable flying object." So based on this explanation (not that it is formal or saying you necessarily must accept it), I can see why one might mistake/interchange the word "UFO" for out-of-this-world-object (or flying "saucer" if you wish).
And no, certainly not any unidentified object ever seen is a flying saucer. Sometimes the "UFO" label is applied to meteors, sometimes the planet Venus -- neither one of which is "flying" so I think the label "UFO" fails and cannot be taken literally.