The exact definition of chronometers and chronographs is given by the International Organization for Standardization (ISO). (Watchmaking vocabulary according to ISO 6426 / 2-1984, which defines the main technical and commercial terms used in the watchmaking industry).
Compliance with the definition of a chronometer is approved by a neutral official body, which carries out the checks on the watch or movement, and issues an official certificate. In Switzerland, each chronometer is unique, identified by a number engraved on its movement and a certificate number issued by the COSC.
The term chronometer is often incorrectly attributed to instruments equipped with a mechanism activated by push-pieces and enabling the measurement of a short period of time. Such an instrument is known as a chronograph or chronoscope, while a chronometer is a high precision watch.
Chronometer : The label given to a precision timekeeping instrument set for various conditions of use (e.g. ISO 3159 for mechanical wristwatch).
Chronograph : A timekeeping instrument with a counter for measuring and displaying time intervals irrespective of the conservation and possible indication of time. It may also include a recording device.
A chronograph alone cannot be certified a chronometer, as the function of a chronograph is to measure short times - the duration of a particular event. At present, there is no standard to certify the precision of a chronograph. On the other hand, high precision watches with a chronograph function can be certified as chronometers.