What it is depends largely on the value one imbues it with (think memetics), often comes down to where you find it (context counts a lot), and how you choose to define its transitory relevance…

ephemera
1398, originally a medical term, from M.L. ephemera (febris) “(fever) lasting a day,” from fem. of ephemerus, from Gk. ephemeros “lasting only one day,” from epi “on” + hemerai, dat. of hemera “day…” Sense extended to short-lived insects and flowers; general sense of “transitory” is first attested c.1639. Ephemeral is from 1576. Ephemeris “table of astronomical calculations” is from 1551. (from Online Etymology Dictionary)

Images: a few bits of ephemera from a nice Flickr collection here.