What is the meaning of the word "o'clock"?

Definitions:

  1. according to the clock, usually used at whole hour intervals. copy to clipboard ADVERB
    • It's three o'clock in Tokyo now. copy to clipboard
  2. Used after a number form one to twelve to say the time when it is exactly that hour. copy to clipboard ADVERB
    • He called me at four o'clock in the morning. copy to clipboard