What does "o'clock" mean?

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, i.e., at the hour.

What is a synonym for "o'clock"?

Common synonyms include: time of day.

O'clock

//əˈklɑk// adv, slang

adv, slang ·Moderate ·High school level

Definitions

Adverb

1
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, i.e., at the hour. not-comparable
"We are expected to be there at six o'clock in the morning!"
2
Used to indicate that it is time to do a specific action, or time for a specific action to occur. humorous, informal, not-comparable
"I'm feeling peckish; I think it must be nearly dinner o'clock."
3
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 on a horizontal plane, or directly up on a vertical plane, and three being to the right on either. not-comparable, with-numeral
"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."