The American Heritage® Dictionary of the English Language: Fourth Edition. 2000.
A.M.
ABBREVIATION:
1.airmail2. or a.m.Latin anno mundi (in the year of the world) 3. also a.m. or a.m.ante meridiem
USAGE NOTE:
By definition, 12 a.m. denotes midnight, and 12 p.m. denotes noon, but there is sufficient confusion over these uses to make it advisable to use 12 noon and 12 midnight where clarity is required.
