UFO is an acronym for Unidentified Flying Object. That's all it means; it does not necessarily imply anything extraterrestrial.
In the purest form, the evidence of a UFO could be no more than the photograph of a strange light in the sky. It could turn out to be the Aurora Borealis or something else that we do know about. Until that final solution is found, though, it is a UFO.
There are genuine photographs and videos the world over that show metallic craft of seemingly extraterrestrial origin. By "genuine," I mean they haven't been proved to be hoaxes or legitimate earthly phenomena.
Fake photos and videos far outnumber the "genuine" articles. And people have been doing this for decades. Digital technology has made it easier to produce false images that are more difficult to disprove. But there remain a number of photos and videos taken in the old-fashioned way (possibly manipulated by overexposure or superimpositions) that cause a divide between believers and skeptics.
Verbal evidence isn't strong enough, of course. If someone claims he saw a UFO, though, I don't automatically discount it. It could be something generic, like a plane, but it might appear otherwise to him because of atmospheric conditions, distance, and other parameters. It is a true UFO because he doesn't know what it is when he sees it.
Many UFO sightings turn out to be military jets and helicopters that the public doesn't yet know about. Others turn out to be cloud formations or some such.
Yet there are many that clearly show a metallic vessel of some kind that the authorities claim is a flock of birds, clouds, or whatever, when it's obviously not natural. After rigorous scrutiny and testing, some of these turn out to be hoaxes of some kind, but a few are determined to be genuine though unexplained.
Some believe that the government's disregard and continual cover-ups of such evidence is evidence in itself for the existence of alien craft. I do see a good point in this, but one should remember that when the government develops top-secret aircraft, it's their job to cover it up in hopes that it won't be leaked to enemies.
However, I have heard some ridiculous stories. I've seen various photographs that have undergone rigorous testing; they show no evidence of doctoring or manipulation, and the subject can't be described as being of earthly origin. Yet the government has written some of these "genuine articles" off as swamp gas, birds reflecting off the water, or clouds . . . even when the photo shows a cloudless sky.
I've also seen videos of small metallic objects conducting maneuvers that no known craft on earth can replicate. Lights do not appear and then speed away with what looks like some kind of exhaust. Birds do not fly backwards, nor do they shine like chrome under the sun. And if it's a bird, sooner or later it will flap its wings. There's no wing-flapping at all in these videos.
What we're seeing in these videos, I can't rightly say, but it appears to be alien craft of some kind. I mean, looking at it logically, what else could it be?
Say you have a photo or a video of something strange in the air. Send it off for heavy analysis and testing. If it's determined that the photo or video hasn't been doctored or manipulated in any way, and the object isn't something we know about, then you can call it genuine evidence of a UFO.
That doesn't clear up any mystery, of course. But for it to be a UFO, genuine or not, it must be unidentified. After all, we're looking for the unknown here, not the known.