A chronometer is a highly accurate mechanical timepiece that has undergone strict precision tests over a period of time in a host of different circumstances. The term "chronometer" was coined by Jeremy Thacker of Beverley, England in 1714, referring to his invention of a clock ensconced in a vacuum chamber. The term is also used to describe a marine chronometer used for celestial navigation and determination of longitude, which was invented by John Harrison in 1730.

Chronometer testing measures the movement of the watch towards a set of accuracy standards, in different positions and at different temperatures. Watch brands use a neutral, independent organization to conduct the individual testing, and only a small percentage of watches on the market today are certified chronometers. In order to receive this certification, the watch must remain within +6 and -4 seconds per day.

It is important to note that a chronometer is not to be confused with a chronograph, which is any watch or clock with a stopwatch function for measuring time. A chronograph is a watch that has a built-in stopwatch feature, which allows it to measure elapsed time.

The history of chronometers can be traced back to the golden age of seafaring exploration in the 18th century, when ships required the use of a highly accurate onboard clock that enabled their navigators to determine longitude in order to avoid the perils of running aground or veering hopelessly off course. The modern chronometer is a large, well-made watch but with a detached chronometer escapement, suspended in gimbals (a set of rings connected by bearings) poised so as to remain horizontal whatever the inclination of the ship.