In conjunction with a numeral, indicates the time within a twelve hour period (midnight to noon or noon to midnight), specifically the time when the hour hand of a clock points precisely to the symbol or marking corresponding to the designated numeral.
- We are expected to be there at six o'clock in the morning!
- It is two o'clock.
- What o'clock is it?
Shortened form of “of the clock” or “on the clock”; that is, “according to the clock”.