What is the definition of meridiem?

Meridian is an imaginary arc on the Earth's surface from the North Pole to the South Pole that connects all locations running along it with a given longitude

That is correct but COMPLETELY unrelated to this question. Meridiem is not the same as meridian. meridiem is latin for "mid-day" as in "post meridiem" or "after mid-day"

AM stands for ante meridiem. Ante is latin for "before" or "prior to" (all you Poker players out there now know why the forced bet at the beginning of the hand is called the ante :)