Ephemeral
Etymology
From Greek ephemeros, "lasting only a day": epi- ("on") + hemera ("day"). It first described fevers and mayflies that ran their course within a day, then broadened to anything short-lived.
adjective · /ih-FEM-er-ul/
Lasting a very short time; fleeting.
What it names
The quality of things that matter partly because they won’t last — a mood, a season, a trend, a message set to disappear.
Example
Social media rewards the ephemeral — posts built to vanish within a day.
Related idea
Ephemeral is about brevity; liminal is about being between states — a thing can be both.