Definition of a.m. abbreviation from the Oxford Advanced American Dictionary

  

a.m.

 abbreviation
(also A.M.)abbreviation
NAmE//ˌeɪ ˈɛm//
 
 
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 between midnight and noon (from Latinante meridiem) It starts at 10 a.m. compare p.m.
See the Oxford Advanced Learner's Dictionary entry: a.m.