1. before noon.
2. the period from midnight to noon, esp. the period of daylight prior to noon: Shall we meet Saturday a.m.?
3. a morning newspaper, sometimes issued shortly before midnight. Cf. p.m.
[ L ante meridiem ]
Usage . The abbreviation A.M. for Latin ante meridiem, meaning "before noon," refers to the period from midnight until noon. One minute before noon is 11:59 a.m. One minute after noon is 12:01 p.m. Many people distinguish between noon and midnight by saying 12 noon and 12 midnight. Expressions combining A.M. with morning ( 6 a.m. in the morning ) and P.M. with afternoon, evening, or night ( 9 p.m. at night ) are redundant and occur most often in casual speech and writing. Both a.m. and p.m. sometimes appear in capital letters, especially in printed matter.