Answer

Oddly enough, this question seems to already have been answered (and then later removed) at StackOverflow.
(this is the last existing Wayback Machine backup link to it.)

• The Unicode standard for date/time formatting calls it “period”.

• Wikipedia says: The 12-hour clock is a time conversion convention in which the 24 hours of the day are divided into two periods

• Ruby documentation (and probably a lot of strftime references) seems to refer them as “Meridiem Indicator”.

From a user interface perspective, a “Period” label is too ambiguous, and “Meridiem Indicator” too pedantic, still leaving “AM/PM” as the best choice.

Aside

As for the actual meanings of AM/PM, they come from the Latin “ante meridiem” (“before noon”) and “post meridiem” (“after noon”).