The Collaborative International Dictionary of English v.0.48:
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.) [1913 Webster]WordNet (r) 3.0 (2006):
antemeridian adj 1: before noon [ant: postmeridian]