A UFO may not be a UFO
As an amateur astronomer and someone who was employed in the Deep Space tracking industry, I've spent many many nights observing and taking photos of the night sky and have never seen anything that wasn't easily explained as a natural phenomena.
That's not to say that UFO's don't exist, they do!
Anything we see flying that we can't readily identify is one, but it aint the same as LGMs (Little Green Men).
Having said that, one can't ever say that spaceships (or the like that people refer to when they call them UFO's) don't exist. It's just that there really isn't very good evidence to support them so far.
As Carl Sagan once said,
"There are a lot of interesting but unreliable sightings out there and there are a lot of reliable but uninteresting sightings also. What I want to see is a interesting AND reliable sighting"
So far, alas, there really aren't very (if any) many that stand up to scientific scrutiny.
The physics also don't readily support the existence as we know them, but then could we recognise them in some other guise?
If they have travelled here under some mechanism that short circuits the limitations imposed by the speed of light/mass relationship, then we may not recognise them anyway, in which case they should be classed as Unseen Flying Objects.
Still UFO's