ephemeral describes something lasting a very short time — and, in its original and still-technical sense, something lasting only a single day.
The general sense is the one most readers meet: an ephemeral trend, an ephemeral fame, a pleasure that is gone before it can be held. Biology keeps the narrower sense, describing plants and insects that complete their whole life cycle in a brief window, and hydrology describes ephemeral streams and ponds that exist only after rain. Software has adopted the word for anything designed to disappear on schedule — ephemeral messages that delete after viewing, ephemeral storage and environments that vanish when a session ends. The noun ephemera names printed matter made for short-term use, such as tickets, posters, and flyers, which collectors often prize precisely because so little of it survives.
The word entered English in the 1560s from the Greek ephēmeros, "lasting only a day," built from epi ("on") and hēmera ("day"). It was first applied to fevers and illnesses that ran their course in a day, broadened to mean transitory in general by the 1630s, and was then extended to short-lived insects and flowers; the mayflies are classed as Ephemeroptera for the same reason.
Ephemeral sits among close neighbors. Transient emphasizes passing through rather than staying, fleeting emphasizes speed and the difficulty of holding on, and evanescent emphasizes fading or vanishing like vapor. Ephemeral stresses the brevity of a thing's whole existence: it is not just passing, it was only ever going to last a moment.
In my work, ephemeral names the problem 📝MythOS answers. Conversations convey things once and lose them, which 📝Interaction vs. Interactive calls wasteful redundancy and 📝Disposable Dialogue names outright; a 📝Living Document is built to evolve instead of expire.