In a 24-hour day, a.m. and p.m. refer to the two 12-hour periods of the day. A.m. refers to the period between 12:00 in the morning and 11:59 in the late morning, whereas p.m. refers to the period between 12:00 midday and 11:59 in the late evening.
The abbreviation a.m. is derived from the Latin phrase ante meridiem, which translates to "before midday." The abbreviation p.m., also derived from Latin, stands for post meridiem, which translates to "after midday."
The time designations a.m. and p.m. are used when referring to 12-hour clocks. The 24-hour clock, which is used in the military, eliminates a.m. and p.m., instead using the numbers 0000 through 2400 to refer to hours and minutes, with the first two numbers representing hours and the last two numbers representing minutes.
The terms coordinate with the rising, setting and midday position of the sun. Before precise timekeeping came into use, people could tell the time of day based on the sun's position. For example, when the sun crosses the meridian in a specific geographic location, it is noon, otherwise known as midday, signifying the end of the a.m. period and the beginning of the p.m. period. People residing in the same time zone experience noon at the same time, but as they move through time zones, the time differs in one-hour increments.