Glossary
Contrôle Officiel Suisse des Chronomètres, the official Swiss agency that tests and certifies individual movements as chronometers before casing.
COSC certifies movements, not finished watches, and issues a certificate per individual movement serial number, which some brands preserve with the watch. A dial marked "Chronometer" should correspond to a movement built to COSC's tolerances, so the absence of expected chronometer markings on a model that should carry them is worth investigating.
Browse the full glossary, or look up a serial number in the registry.