O'clock (adverb)

Used after a number from one to twelve to indicate the hour of the day.

Origin:

Mid 18th century: abbreviation of of the clock.

Examples:

1. We're meeting at six o'clock.
2. The store opens at nine o'clock in the morning.
3. He usually goes to bed at ten o'clock.
4. The movie starts at seven o'clock sharp.
5. The train arrives at three o'clock in the afternoon.