PM (also written P.M., pm, p.m.) is an abbreviation that stands for post meridiem, which is Latin for "after midday." It includes times that we may refer to as afternoon, evening, or night.
The use of the abbreviation PM or AM is necessary when describing time using a 12-hour clock. This is because they distinguish the hours between noon (12:00 PM or 12:00 on the 24-hour clock) and midnight (12:00 AM or 00:00 on the 24-hour clock) from those between midnight and noon. In the 12-hour system, not writing an AM or PM would make it unclear which hour is being referred to. For example, if someone said "meet me at the theater at 9:00," it would not be clear whether you would meet at the theater at 9:00 at night or 9:00 in the morning, hence the need for the designation of PM and AM.
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A sundial is an instrument that shows the time of day by the shadow of a pointer on a flat surface. Sundials were used to keep time before mechanical clocks were invented. This photo shows a "human sundial" installed in a schoolyard in Lafayette, Louisiana.