I don't believe in alien space craft visiting earth. I think it unlikely that an advanced civilization will have a need for craft that we would recognize as craft long enough that we are statistically likely to encounter them.
So, when I say UFO, I just mean something that flies and I don't know what it is. Possibly an illusion, or a combination of a physical effect and malfunctioning eyesight.
Two days ago, while out walking late at night, I saw something streak across the sky, heading southeast over my home town. It went from unseen to lit (but not very brightly, less so than a full moon) in about two seconds, and then back to unseen in about one second, and in those three seconds covered a large percentage of my field of view. It was large enough to see cross-sectional size, and not just a point of light, perhaps half the width of the moon, so fairly large, depending on how close it was. It looked like a streak, with the outer edges of the streak brighter than the center. It was not ill-defined or breaking apart, like a meteor, and it didn't light up the sky around it. It was crisp and straight. If it was high enough altitude to account for it being silent, then it was REALLY frickin' large and fast, or if it was close enough to account for its relative speed and size, then it was REALLY frickin' silent. I heard nothing at all.
I said, "What the hell was that?!", and stared for quite a while. I waited for a sonic boom, or an explosion in the distance, and heard nothing. I went over in my mind what I'd seen, and then went on with my walk, because what the hell else are you going to do about it?