o'clock

adverb

1. of, by, or according to the clock (used in specifying the hour of the day): It is now 4 o’clock.
2. 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.

adverb

1. used after a number from one to twelve to indicate the hour of the day or night
2. used after a number to indicate direction or position relative to the observer, twelve o’clock being directly ahead or overhead and other positions being obtained by comparisons with a clock face

adj.c.1720, abbreviation of of the clock (1640s), from Middle English of the clokke (late 14c.). Use of clock hand positions to describe vector directions or angles is from late 18c.