It's something up in the sky, that you can't identify. I've seen plenty, and if you've ever seen something flying that you can't identify then you've seen them too.
I kind of wish Unidentified Flying Object hadn't been conflated with space aliens.
The stereotypical description is a circular object that can hover and make sharp directional changes. Usually they're described as having lights and sometimes as having no seams or visible rivets.
I kind of wish Unidentified Flying Object hadn't been conflated with space aliens.