six o'clock

(In both the 12-hour clock and the 24-hour clock) The start of the seventh hour of the day; 6:00 a.m. (06:00).

(Only in the 12-hour clock) The start of the nineteenth hour of the day; 6:00 p.m. (18:00).

(informal) A position behind (horizontal clock orientation) or below (vertical clock orientation) (from the location of the 6 mark on a clock face)