o'clock

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adverb Of or according to the clock.
• adverb According to an imaginary clock dial with the observer at the center and 12 o'clock considered as straight ahead in horizontal position or straight up in vertical position. Used to indicate relative position.

from The Century Dictionary.

• noun See clock.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adverb 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.
• adverb 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.
• adverb In conjunction with a numeral, indicates the direction, relative to the speaker, 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.
• adverb Used to indicate direction on a plane, often vertical with 12 o'clock being straight up, representing a projection of the speaker’s field of vision.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adverb according to the clock