Contrôle Officiel Suisse des Chronomètres (Official Swiss Chronometer Testing Institute, "COSC") is a neutral and independent body that tests mechanical movements for high-precision accuracy tolerances in timekeeping.

Testing relates to the mechanical movement itself, only, as opposed to any given timepiece with that movement installed.

During this test, movements are monitored for 360 consecutive hours (15 days), under 3 different temperature conditions and with the watch resting in 5 different orientations.

Although "chronometer" is generally defined in your Webster's Dictionary, Alan Downing notes in his TimeZone article, "Inside COSC":

"...COSC's very survival depends on the definition enshrined in the international standard — a 'precision watch, rated in different temperatures and positions and which has obtained an official rating certificate.'"

That said, Omega merely notes that the title "chronometer" refers to a mechanical watch with an average daily variation rate of between not more than -4 and +6 seconds per day.