Fleeting is an adjective that describes something that happens really fast, or something that doesn’t last as long as you’d like.
Driving in a car on the highway, you see a unicorn in the woods, but you only get a fleeting glimpse of it because you’re driving too fast. Bummer. Fleeting comes from the Old English word flēotan, which means “float, swim.” Like a ghost ship floating by on a foggy night, fleeting things disappear as fast as they appear. Fleeting love may last more than a moment, but it won’t stay for very long, and that’s why it’s called fleeting.