"Meridiem" is noon, AM is "Ante meridiem" (before) and PM is "Post meridem" (after). So noon is officially 12:00 m., and midnight is 12 pm.
Some use 12 n. and 12 m.
A wise UI designer will use noon and midnight to avoid confusion.
Source: A Handbook for Scholars, Mary-Claire van Leunen, Knopf press, pg 123.