I know that AM/PM is for ante/post meridiem, but what is it actually called? Meridian indicator? 12 hour indicator? Something way more clever?
Oddly enough, this question seems to already have been answered at StackOverflow. The distinction is called the period.
As for the actual meanings of AM/PM, they come from the Latin "ante meridiem" ("before noon") and "post meridiem" ("after noon").