- a.mAs an abbreviation of Latin ante meridiem ‘before noon’, a.m. is pronounced as two letters and written in the form 8.15 a.m. (or am; in AmE 8:15 a.m.). Note that 12.00 a.m. is midnight and 12 p.m. is midday; because of the uncertainty these designations cause, the explicit forms 12.00 midnight or 12.00 midday are often to be preferred. The abbreviation is sometimes used informally as a noun: I arrived here this a.m.
Modern English usage. 2014.