ante meridiem

…as am, which stands for ante meridiem, “before midday” in Latin, while the second is referred to using pm, meaning post meridiem, or “after midday.” This nomenclature is based on the position of the Sun in relation to the meridian. In writing, periods can be added or omitted and lowercase…

The abbreviation am stands for ante meridiem, “before midday” in Latin, while pm means post meridiem, or “after midday.”