a.m. Look up a.m. at Dictionary.com
also a.m., 1762 in reference to hours, an abbreviation of Latin ante meridiem "before noon" (q.v.). Synonymous with "morning" by 1776. AM as a type of radio wave broadcast, 1921, abbreviation of amplitude modulation (see amplitude). Affixed to a name, an abbreviation of artium magister "Master of Arts," an abbreviation preferable in a purely Latin idiom; British English prefered M.A. In some old chronologies, a.m. is anno mundi "year of the world."