Etymology
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a.m.

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 preferred M.A. (magister artium, 1730). In some old chronologies, a.m. is anno mundi "year of the world."

updated on October 24, 2018

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Definitions of a.m. from WordNet
1
A.M. (adv.)
before noon;
Synonyms: ante meridiem
2
A.M. (adj.)
before noon;
Synonyms: ante meridiem
From wordnet.princeton.edu, not affiliated with etymonline.