An escapement is a device in mechanical watches and clocks that transfers energy to the timekeeping element (the "impulse action") and allows the number of its oscillations to be counted (the "locking action"). The impulse action transfers energy to the clock's timekeeping element (usually a pendulum or balance wheel) to replace the energy lost to friction during its cycle, to keep the timekeeper oscillating. The escapement is driven by force from a coiled spring or a suspended weight, transmitted through the timepiece's gear train. The amount of stored energy, energy loss and efficiency of transfer to the timekeeping element determines the time a clock will run after it has been wound. The escapement releases the tooth of a gear, which therefore changes from a "locked" state to a "drive" state until the opposite arm strikes another tooth on the gear, which locks the gear again. A clock's tick is the sound of the gear train stopping as the escapement locks. The gear train is accelerated and decelerated with each tick of the clock. This locking action of the escapement allows each cycle of the timekeeping element to be counted. During each cycle the escapement permits a gear train to advance or escape slightly. The periodic advancement results in moving the timepiece's hands forward at a steady rate. This starting and stopping accounts for most of the energy usage from the spring or weight when a clock is in good working order.

Read more about Escapement:  History, Reliability, Accuracy, Mechanical Escapements, Electromechanical Escapements