1. The terms "A.m." and "p.m." are abbreviations of the Latin ante meridiem (before midday) and post meridiem (after midday)

2. Footnote on "Meridiem" vs "Meridian" Should "Am" be "ante meridiem" or "ante meridian" (likewise for PM)?

3. The official (according to an American, Australian and British dictionary I checked), and most common spelling for Am is "ante meridiem" which is a Latin phrase.I recommend that spelling!