A group of two or more gears joined together. If the centre gear is used only to change the direction of rotation or to act as a "spacer", it is called an idler gear.
(n) A mechanism consisting of two or more gears and used to transmit power. Normally, gear trains are used to do one or more of the following: increase speed, decrease speed, or change the direction of motion.
The gears used in a mechanical watch which run from the mainspring which powers the watch through to the escapement which translates that power into timekeeping.