merīdiēs

Merīdiēs in Latin means midday, noon. It comes from medius (middle) and dies (day) — the middle of the day. The contracted form meridies replaced the full medidies early in Latin. The abbreviations a.m. (ante meridiem, before midday) and p.m. (post meridiem, after midday) divide the 12-hour clock into two halves. The convention is Roman. The abbreviations are Latin. The clock face is universal.

The 12-hour clock, divided at noon and midnight, was the standard in European timekeeping from the medieval period. The Romans used a 12-hour daytime system (sunrise to sunset) that varied with the season. The fixed 12-hour system — identical hours regardless of daylight — came with mechanical clocks in the fourteenth century. A.m. and p.m. were the labels that told you which half of the day the clock referred to.

The 24-hour clock — used by militaries, most of continental Europe, and international aviation — eliminates the need for a.m. and p.m. entirely. 1:00 p.m. becomes 13:00. The ambiguity vanishes. But the 12-hour clock persists in the United States, the United Kingdom, Australia, and several other countries. In these countries, Latin abbreviations from the Roman Empire appear on every digital clock, every schedule, every alarm setting.

The moment of noon itself is neither a.m. nor p.m. — it is the meridiem, the midpoint. Strictly speaking, 12:00 noon is neither 'before midday' nor 'after midday.' The convention to call it 12:00 p.m. is widespread but logically indefensible. Midnight has the same problem. The Latin system was designed for everything except the moment it is named for.