Evanescent
Etymology
From Latin evanescere, "to disappear, vanish," built from ex-, "out," and vanescere, "to vanish," which traces back to vanus, "empty" (the same root behind "vain" and "vanity"). The picture is of something emptying out into nothing. English took it up in the eighteenth century for things too delicate to hold onto.
adjective · /ev-uh-NES-uhnt/
Vanishing quickly, like a scent or a wisp of vapor that fades almost as soon as it appears.
What it names
A word for the kind of fleetingness that is felt as it happens — a fragrance, a blush, a chord dying away — where the vanishing is part of the beauty.
Example
Happiness, she thought, was evanescent by design — you noticed it most in the instant it began to slip away.
Related idea
Transient simply states that something is short-lived; evanescent dwells on the delicate act of fading and usually carries a note of beauty or regret.