1. In horlology, an exceptionally precise watch, timepiece, or clock: The time of day is indicated by a chronometer set to Greenwich Mean Time.
2. An instrument for measuring time, specifically applied to time-keepers adjusted to keep accurate time in all variations of temperature: Chronometers are said to differ from watches by having a more perfect escapement and a compensation balance, and are used for determining longitude at sea, and for other exact observations.