What is O’clock?

O’clock definition and meaning on Dictionary terms:
of, by, or according to the clock (used in specifying the hour of the day): It is now 4 o’clock.
according to a method for indicating relative position whereby a plane in space is considered to be numbered as a clock’s face, with 12 o’clock considered as directly ahead in horizontal position or straight up in vertical position.


reference: https://www.dictionary.com/browse/o-clock