In general, any watch can be called "chronometer", because they eventually are time measuring devices. However, in horology this term refers to the watch movement with the utmost precision.
Nowadays watches can be called chronometers when (and only with this condition) they have passed special tests conducted by the Swiss Official Chronometer COSC Institute.
COSC - Contrôle Officiel Suisse des Chronomètres – was founded in 1973 by five Swiss cantons (Bern, Geneva, Neuchâtel, Solothurn and Vaud) as a not-for-profit association.
The BOs (short for "Bureaux Officiels de Contrôle de la marche des montres" - meaning Official Watch Rating Centres) currently constitute the three laboratories within the COSC. Located in Bienne, Le Locle and St-Imier, their mission is to test the movements submitted by manufacturers. They have each earned individual accreditation as SCS (Swiss Calibration Service) laboratories from SAS (Swiss Accreditation Service).
The three BOs work with state-of-the-art equipment developed in-house by the COSC engineers. The specific requirements are such that all the BO instruments have had to be custom-made, since nothing equivalent exists on the instrument market.
The COSC management, based in La Chaux-de-Fonds, coordinates the operation of the BOs and provides them with the measurement and result management equipment, as well as ensuring maintenance.
It establishes the testing prescriptions applicable to the various types of movement submitted, it develops the equipment and measurement methods for the BOs and undertakes any necessary action in matters relating to marketing, communication and defence of the chronometer in the broadest sense of the term.Should be noticed
Watch owners should remember that the COSC certificate is not a precise indicator of how well a watch will perform on the person's wrist. The test results show how well a given movement performed over a given period of time under certain conditions. Several factors can affect the actual performance the owner observes. First, The COSC test is only an artificial approximation of actual use. Second, the COSC does not test "watches" but only uncased movements. Following the test (assuming a movement passes), the movement returns to the manufacturer, where it is either cased or placed in a vault for casing at a later date. Movements can spend significant time in the vault. When they do, they are usually disassembled, cleaned, re-assembled, oiled and regulated, then cased. This can certainly affect the movement's performance, yet re-testing is not required.
Next, the watch may be affected by shipping in the distribution chain, where it might be exposed to shocks or extreme temperatures. Conditions at the dealer's shop also affect performance. If the watch sits unused for many months, if it sits under hot lights or in the window in the sun, if customers drop it while trying it on, this will affect how the watch performs on the owner's wrist.
Finally, the owner's personal habits can affect performance.