Meridiem

Meridiem is a term used to indicate the portion of the day relative to local solar noon. It is employed in the 12-hour clock to distinguish morning from afternoon. The word is formed from the Latin terms ante meridiem “before noon” and post meridiem “after noon.”

Etymology and meaning: Meridiem derives from meridies, meaning noon. Ante meridiem and post meridiem specify the two halves of the day around the central point of solar noon. In modern time notation, these concepts are usually abbreviated as AM and PM in English.

Usage and notation: The meridiem concept is associated with the 12-hour clock, where the day is split into two 12-hour cycles. The 24-hour clock, by contrast, does not use meridiem and represents time using a continuous 00:00–23:59 scale. In practice, many languages and styles use AM/PM or a.m./p.m., while others rely solely on the 24-hour format.

Ambiguities and conventions: Times around noon and midnight can be confusing, with 12:00 a.m. commonly interpreted as midnight and 12:00 p.m. as noon. Some style guides avoid AM/PM altogether to reduce this ambiguity, opting for 24-hour notation or clarified phrases such as “12:00 noon” and “12:00 midnight.”

History and variations: The 12-hour system with meridiem notation emerged in Europe with the spread of mechanical clocks and became widespread in English-speaking regions in the early modern period. While English commonly uses AM/PM, many languages either adopt the 24-hour system or use localized terms to indicate morning, afternoon, and evening without relying on a.m./p.m.