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Houghton Mifflin

P.M. also p.m. or p.m.

abbr.
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.


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