Not so sure what meaning of UFO you have other than "Unidentified Flying Object" which I am used to describe "intelligent aliens who have a physical presence on or near Earth" (don't believe in them) vs. "intelligent aliens who live somewhere else" (do believe in them)

Of course Jacques Valee thought it was more likely they were time or dimension travelers

https://www.amazon.com/Messengers-Deception-UFO-Contacts-Cul...