A clock is a device used to measure, verify, keep, and indicate time. The clock is one of the oldest human inventions, meeting the need to measure intervals of time shorter than the natural units: the day, the lunar month, and the year. Devices operating on several physical processes have been used over the millennia.

What does o'clock mean?

o'clock. / (əˈklɒk) / adverb. used after a number from one to twelve to indicate the hour of the day or night. used after a number to indicate direction or position relative to the observer, twelve o'clock being directly ahead or overhead and other positions being obtained by comparisons with a clock face.

Where does the o in o'clock come from?

An apostrophe is used in o'clock because the word is a contraction of the phrase "of the clock." Just like other contractions, the apostrophe takes the place of missing words or letters from a longer word or phrase.