Before midday; applied to the hours between midnight and the following noon. Usually abbreviated a.m., a.m., or am.
ante meridiem/ˌanti mɪˈrɪdɪəm/ /ˌanteɪ mɪˈrɪdɪəm/
Mid 16th century; earliest use found in John Dee (1527–1609), mathematician, astrologer, and antiquary. From classical Latin ante merīdiem before midday from ante before + merīdiem, accusative of merīdiēs noon.