The machinery inside a watch that keeps things ticking is called the movement or a calibre. A synonym of size, the word ‘Calibre’, first used as a watchmaking term in 1715, originally referred to the build of a watch movement—the layout, dimensions, shape and size of the wheels, barrels, bridges and so on. Today the word is a substitute for movement,

the complete mechanism with the mainspring, escape wheel, bridges, gear train, and other components, including the rotor in automatic watches. Calibres are either mechanical or quartz-based. While the latter is powered mostly by a battery, the former can either be manual winding, or self-would by an oscillating mass or rotor.