The Contrôle Officiel Suisse des Chronomètres is the institute tasked with testing and certifying Swiss watches for accuracy. Those that pass, are referred to as “chronometers”.
In terms of accuracy, it’s normal for mechanical watches to deviate between 10 to 20 seconds, out of 86,400 seconds in a day. Watches that bear the label “COSC certified chronometer” means the movement has passed COSC standard for accuracy, deviating within -4/+6 seconds a day. Digital watches generally deviate by a second a day, or less.
Watches with a stopwatch function. Besides regular chronographs, there are also flyback chronographs that allow for quick timings of successive events; and rattrapante or split seconds chronographs for recording lap times.