o'clock

[ uh-klok ]
/ əˈklɒk /

adverb

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.

Origin of o'clock

First recorded in 1710–20; see origin at o', clock1
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

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British Dictionary definitions for o'clock

o'clock

/ (əˈklɒk) /

adverb

used after a number from one to twelve to indicate the hour of the day or night
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

Word Origin for o'clock

C18: abbreviation for of the clock
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012