A certified chronometer is a high-precision watch capable of displaying the seconds and which has been tested and certified by an official neutral body (Contrôle Officiel Suisse des Chronomètres - COSC).
Each chronometer is unique, identified by a number engraved on its movement and a certification number given by the COSC. Each movement is individually tested for several consecutive days, in 5 positions and at 3 temperatures. Each movement is individually measured. Any watch with the denomination "chronometer" is provided with a certified movement.
The term chronometer is often wrongly applied to timekeeping instruments fitted with an additional mechanism that may be set in motion by pushbuttons to enable measurement of the duration of an event. Such an instrument is in fact a chronograph or chronoscope. It may of course be chronometer-certified, provided it meets the criteria set for the standard.