Longman Dictionary of Contemporary English homepage

Date: 1400-1500
Origin: of the clock

o'clock

adverb
     
o'clock S1 W3

one o'clock/two o'clock etc

TMC one of the times when the clock shows the exact hour as a number from 1 to 12:
It's already 9 o'clock.
The meeting is at 10 o'clock.
O'clock O'clock

Do not use o'clock for times that include minutes or parts of an hour ten past nine (NOT ten past nine o'clock) I get up at 6.30 (NOT 6.30 o'clock) Do not use o'clock when the time is written in the form 1.00, 2.00, 3.00 etc. Write 1 o'clock, 2 am, or simply 3.00.

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