What Do am and pm Stand For?

The abbreviations am and pm are used in the 12-hour clock system, which divides the day into two 12-hour periods, distinguishing between the morning and afternoon or evening hours. This system is based on the position of the Sun relative to the meridian, an imaginary north–south line on the Earth’s surface that connects both geographic poles. The abbreviation am stands for ante meridiem, “before midday” in Latin, while pm means post meridiem, or “after midday.” 

The concept of a 12-hour clock has been around since ancient times. The Babylonians and ancient Egyptians were among the early adopters of this timekeeping method, using celestial bodies to track time. The use of am and pm, however, is not a relic of the past; it remains prevalent in many English-speaking countries today. Despite the global spread of the 24-hour clock, in which the day begins at 00:00 and ends at 23:59, the 12-hour system persists in such places as the United States, India, and Australia. This is partly due to the convenience of dividing the day into two easily digestible halves.

While the 12-hour clock can sometimes lead to confusion—especially when am or pm is omitted— it is a system that many find intuitive and easy to use. However, it’s important to be clear when communicating times to avoid misunderstandings, particularly around the ambiguous 12:00 hours. When am or pm are omitted, time expressions such as “in the morning,” “in the afternoon,” or “at night” are often included by those using the 12-hour clock.