Fleeting
Etymology
From Old English fleotan, "to float, drift, swim," the same family that gives us "float" and, more distantly, "flee" and "flit." The idea moved from drifting on water to swiftly slipping past, so that by the sixteenth century "fleeting" meant "quickly passing away." The image underneath is of something carried off before you can hold it.
adjective · /FLEE-ting/
Passing very swiftly and gone almost at once.
What it names
The everyday word for the briefest kind of passing — a fleeting glance, a fleeting thought — something that barely lasts long enough to register.
Example
For a fleeting moment their eyes met across the platform, and then the train pulled him away.
Related idea
Ephemeral frames brevity as a fixed lifespan — lasting a day; fleeting frames it as motion, something moving past too fast to catch.