- or A.M.1) hor before noon2) hor the period from midnight to noon, esp. the period of daylight prior to noonCompare p.m. Etymology: 1755–65; < L ante merīdiem usage: The abbreviation a.m. refers to the period from midnight until noon. One minute before noon is 11:59 a.m. One minute after noon is 12:01 p.m. Many people distinguish between noon and midnight by saying 12 noon and 12 midnight. Expressions such as 6 a.m. in the morning and 9 p.m. at night are redundant.
From formal English to slang. 2014.