ephemeron

An ephemeron is something short-lived or transitory—existing only briefly before passing away.

The word typically describes insects that live for just a day (mayflies are the classic example), but extends to anything fleeting: a cultural fad, a passing sensation, a piece of writing meant only for the moment.

The plural is usually ephemera, which has become the more common form, especially for collectible paper items like old tickets, postcards, and advertisements.