The dictionary definition of a chronometer (from the Greek "chronos" for time and "metro" for measuring instrument) is a device for measuring the time. In the world of watchmaking, however, it has come to refer to an especially precise timepiece that has received some form of official certification.
Englishman Jeremy Thacker first coined the term in 1714 to describe a clock that he had housed inside a vacuum chamber. In 1778, another Englishman, John Arnold, was the first to use the term to describe the exceptional timekeeping properties of his pocket watch No. 36. At the time, precision timekeeping was important for the calculation of longitude, which countries were competing to do ever more accurately for dominance in ocean navigation.
Nowadays the use of the term "chronometer" is governed by the international standard ISO 3159:2009, which includes the different categories of chronometers, the testing procedures and the minimum requirements.
In Switzerland, the independent body responsible for certifying chronometers in accordance with this standard is the COSC. The COSC subjects the watch movement to 15 days of tests in different positions and at different temperatures and includes the function of any additional complications. Over the first ten days of testing, the movement's precision must remain within a tolerance of -4/+6 seconds per day in order to be awarded a chronometer certificate. Given that there are 86,400 seconds in a day, this is a phenomenal level of accuracy for a purely mechanical mechanism.