A chronometer watch is a highly accurate timepiece that has been tested and certified to meet certain precision standards set by independent organizations, such as the Swiss Official Chronometer Testing Institute (COSC).
To earn the title of "chronometer," a watch must pass a series of tests that measure its accuracy in various conditions. The COSC, for example, tests watches in five different positions and at three different temperatures over a period of 15 days. The watch must maintain an average daily rate of -4 to +6 seconds to pass.
Chronometer watches are typically mechanical and often used in professional and scientific settings where precise timekeeping is critical. Some luxury watch brands, such as Rolex, Omega, and Breitling, are known for producing high-quality chronometer watches.