meridiem

noun2

1. •
   (rare) Midday, noon.

   • Quantities and frequencies are distributed as follows (neglecting the meridiem hour):[…] — 1919
2. •
   The property of a timestamp of being either a.m. or p.m.

   • The terminal responds N HH MM CR where N, HH, and MM are the meridiem, hour, and minutes, as above. — 1986