1. Literally, at or very near midday (noon). It's only 9 AM and the heat out here is sweltering; by high noon, that sun will be deadly.

2. The peak or most advanced stage of a given period. At the high noon of the economic boom, people spent money on the most ridiculous things. The actor had been a household name at the high noon of his career, but nowadays, there are few who even remember him.

3. The critical or appointed time of a decisive confrontation, crisis, or contest. It's high noon for these two teams, as the result of this game will determine who makes it to the finals.