A clock is a device used to measure, keep, and indicate time. There have been many different types of clocks (or time-measuring devices) throughout history, such as sundials, water clocks, mechanical clocks, spring-driven clocks, atomic clocks, and more.

A number of different types of clocks are still used today, which have varying degrees of accuracy, but for everyday purposes, we mostly just need to be familiar with analog and digital clocks.