Source : Webster's Revised Unabridged Dictionary (1913)

Antemeridian \An`te*me*rid"i*an\, a. [L. antemeridianus; ante +
   meridianus belonging to midday or noon. See {Meridian}.]
   Being before noon; in or pertaining to the forenoon. (Abbrev.
   a. m.)

Source : WordNet®

antemeridian
     adj : before noon [ant: {postmeridian}]