Definition of "o'clock"

o'clock

/əˈklɒk/

adverb
not comparable
1. (modifying a numeral, one to twelve) In conjunction with a numeral, indicates the time within a twelve-hour period (midnight to noon or noon to midnight), specifically the time when the hour hand of a clock points precisely to the symbol or marking corresponding to the designated numeral.
  Quotations
  I say, Watson, what o'clock is it?
  1892, Sir Arthur Conan Doyle, The Adventures of Sherlock Holmes‎, HTML edition, The Gutenberg Project, published 2011
  By one o'clock the place was choc-a-bloc. […] The restaurant was packed, and the promenade between the two main courts and the subsidiary courts was thronged with healthy-looking youngish people, drawn to the Mecca of tennis from all parts of the country.
  1935, George Goodchild, chapter 5, in Death on the Centre Court
2. (humorous) In conjunction with a term representing an action or event that occurs daily, indicates the time that said action or event occurs, or first occurs.
  Quotations
  That same evening at tea-time — (I am sorry to have to introduce you to another eating-scene, but the hours in English households are usually marked by repasts. It is a daily calendar of feasts — breakfast o’clock, dinner o’clock, &c., […] ).
  1880, Henrietta A. Duff, Honor Carmichael, page 251
  “My sister requires your attendance at supper o’clock this evening — no excuse accepted.”
  1904, George Augustus Sala, Edmund Hodgson Yates, Temple Bar, volume 129, page 144
  “It's lunch o’clock. Wanna go out to eat?”
  1998, Carolyn Greene, Heavenly Husband, page 129
3. (with a numeral, one to twelve) In conjunction with a numeral, indicates the direction, relative to the speaker or a vehicle, especially an aircraft, corresponding to the direction the hour hand is pointing at the time corresponding to the numeral, with twelve representing directly ahead and three being to the right.
  Quotations
  It's at nine o'clock about two feet in from the edge of the branch. You need to know your "o'clocks" (nondigital) to spot birds.
  1989, Aileen R. Lotz, Birding around the year: when to find birds in North America
  Without moving your head, move your gaze upward to look at twelve o’clock. Now look down at six o’clock. Repeat three times, then blink several times, and rest with your eyes closed.
  1992, Herbert Benson with Eileen M. Stuart and Staff of the Mind/Body Medical Institute of New England Deaconess Hospital and Harvard Medical School, The Wellness book, page 86
  Thus, you could tell them to "look at three o'clock," or "look just off-center toward nine o'clock," and so forth. Alternatively, in some microscopes a thin black line appears to cut across the field of view
  1994, Peter Abramoff with Robert G. Thomson, Laboratory outlines in biology--VI, page 22
  Three o'clock and nine o'clock would be at the outer right and outer left sides of the tree, respectively, and so on. The clock technique is a very helpful way for one bird watcher to direct others to a hard-to-spot perched bird.
  1997, Bill Thompson, Bird watching for dummies
  At 1057 we were just over the islands and at 1100 the tail gunner reported flak at six o’clock, below.
  1943 November 29, “Target: Germany”, in Life, page 80