What is a Chronometer?
More than just an instrument to tell time, a chronometer is a designation given to a timepiece that has been tested and certified to meet precision standards.
The COSC (Official Swiss Chronometer Testing Institute) is an independent body that is responsible for testing the performance of high-grade watch movements in Switzerland. Each watch submitted undergoes tests for several consecutive days, testing in five positions and three temperatures to determine accuracy. In order to earn the chronometer standards, a movement's performances must meet seven criteria, including a daily second variation ranging from -4/+6 second per day. Only COSC certified watches may use the word 'chronometer' on them in Switzerland.
What a chronometer is not, is a chronograph: a watch with a stopwatch function. A chronograph could be a chronometer, if tested and certified accordingly.
Breitling, the only watch company in the world to have all of their timepieces COSC certified, engraves the COSC number on the caseback of each watch so you know you are purchasing a high precision watch. Oris also has a number of COSC chronometers in its line, including the Oris Artix Chronometer.
A group of Breitling retailers being trained in the Breitling Chronométrie in La Chaux-de-Fonds, where all Breitling caliber movements are made