What does the AM & PM stand for when telling time?
A.M. before midday (ante meridiem) P.M. also p.m. or P.M. abbr. after midday(post meridiem) USAGE NOTE By definition, 12 A.M. denotes midnight, and 12 P.M. denotes noon, but there is sufficient confusion over the meanings of A.M. and P.M. when the hour is 12 to make it advisable to use 12 noon and 12 midnight where clarity is required.