a.m.

"of morning, before mid-day," 1560s, Latin, literally "before noon," from ante "before" (from PIE root *ant- "front, forehead," with derivatives meaning "in front of, before") + accusative of meridies "midday, noon" (see meridian). The adjective antemeridian is attested from 1650s.

1540s, "state or quality of being ample," from French amplitude or directly from Latin amplitudinem (nominative amplitudo) "wide extent, width," from amplus "large, spacious" (see ample). Amplitude modulation in reference to radio wave broadcast (as opposed to frequency modulation) is first attested 1921, usually abbreviated A.M. Related: Amplitudinous.

1550s, "state of being crowded" (now obsolete); 1640s, "fact of occurring often;" from Latin frequentia "an assembling in great numbers, a crowding; crowd, multitude, throng," from frequentem (see frequent). Sense in physics, "rate of recurrence," especially of a vibration, is from 1831. In radio electronics, frequency modulation (1922, abbreviated F.M.) as a system of broadcasting is distinguished from amplitude modulation (or A.M.).