A chronometer is a timekeeper precise enough to be used as a portable time standard, usually in order to determine longitude by means of celestial navigation.

Until the mid 1770s, navigation at sea was an unsolved problem due to the difficulty in calculating longitudinal position.  Navigators could determine their latitude by measuring the sun's angle at noon.  However, to find their longitude, they needed a portable time standard that would work aboard a ship.  The purpose of a chronometer is to keep the time of a known fixed location, which can then serve to measure the difference between zero longitude and your longitude.