AM- Anti Meridian

PM- Post Meridian

AM is used to represent time from midnight 12.00 to noon 11.59.

PM is used to represent time from noon 12.00 to midnight 11.59.

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Am and pm.

am stands for ante meridiem which means before midday
pm stands for post meridiem which means after midday

meridiem is a Spanish term which means midday. Since our day is composed of 24 hours. It is is divided into two 12 hours.

am ranges from 12:00 midnight to 12:00 noon
pm ranges from 12:00 noon to 12:00 midnight