Both AM and PM come from Latin. AM stands for Ante Meridiem and PM stands
for Post Meridiem. Furthermore, Ante Meridiem means before midday (before
Noon), and Post Meridiem means after midday (after noon).
Although not the correct acronym, for easy remembering, you
could say that AM stands for After Midnight and PM stands for
Pre-Midnight or prior to midnight.
Other incorrect but useful way of remembering it is At Morning
for AM and Post Morning for PM.
Also note that it is incorrect to say 12:00 AM or 12:00 PM
because it is not technically before or after 12:00. You may
be better off saying noon and midnight to be correct.
Depending on what style guide you use, there are many correct ways of
shortening Ante Meridiem and Post Meridiem into acronyms or initials: AM and
PM, A.M. P.M., am and pm, a.m. and p.m.
We like to be correct, so when someone asks us what time it
is, we usually answer something like 5 Ante Meridiem or
9 Post Meridiem. Yeah, we are time geeks here
at Research Maniacs!