As 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 American English
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.