n. A fever which lasts but a day or a very short period.n. [capitalized] [NL.] In entomology, the typical genus of May-flies or day-flies of the family Ephemeridæ, having three long caudal filaments.n. A May-fly, day-fly, or shad-fly; an ephemerid. See Ephemeridæ and May-fly.n. Anything very short-lived.n. Plural of ephemeron.