evanescent
ev-uh-NES-untadjMeaning quickly fading from sight or memory
Memory hook e-VANISH-ent: blink, and it is gone. Each firefly glows for a moment, then vanishes into the dark before Zoe can point to it. Their light is evanescent.
The evanescent glow of the fireflies flickered across the meadow.
In context Like breath on a cold window, her smile was evanescent: it appeared for an instant and faded before anyone could be sure they had seen it.
Similar fading, vanishing, fleeting, ephemeral
Opposite permanent, lasting, enduring
From Latin evanescere, “to vanish”: e (out) + vanescere (to fade away), from vanus, empty.