Learner's definition of A.M.
in the morning
— used with numbers to show the time of day
She woke up at 6 a.m. and didn't go to bed until midnight.
The class is held from 11:30 a.m. to/until 2:15 p.m.
12 a.m. [=midnight, one hour after 11 p.m.]
◊ The abbreviation a.m. stands for the Latin phrase ante meridiem, which means “before noon.”