Nouns

(n)Master of Arts, MA, Artium Magister, AMa master's degree in arts and sciences
(n)amplitude modulation, AMmodulation of the amplitude of the (radio) carrier wave
(n)A.M., a.m., AM, amthe period between midnight and noon“Made in the A.M.” (Title of album by One Direction)

Adverbs

(r)ante meridiem, A.M., a.m., am, AMbefore 12 noon“let's meet at 11 A.M.”