- of or according to the clock: nine o'clock at night
- as if on a clock dial, with the number 12 straight ahead or directly overhead: used to indicate direction, esp. of an approaching aircraft
- Of or according to the clock: three o'clock.
- According to an imaginary clock dial with the observer at the center and 12 o'clock considered as straight ahead in horizontal position or straight up in vertical position. Used to indicate relative position: enemy planes at 10 o'clock.
Origin of o'clockShort for of the clock.