The Collaborative International Dictionary of English v.0.48:

Escapement \Es*cape"ment\, n. [Cf. F. ['e]chappement. See
   Escape.]
   1. The act of escaping; escape. [R.]
      [1913 Webster]

   2. Way of escape; vent. [R.]
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            An escapement for youthful high spirits. --G. Eliot.
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   3. The contrivance in a timepiece which connects the train of
      wheel work with the pendulum or balance, giving to the
      latter the impulse by which it is kept in vibration; -- so
      called because it allows a tooth to escape from a pallet
      at each vibration.
      [1913 Webster]

   Note: Escapements are of several kinds, as the vertical, or
         verge, or crown, escapement, formerly used in watches,
         in which two pallets on the balance arbor engage with a
         crown wheel; the anchor escapement, in which an
         anchor-shaped piece carries the pallets; -- used in
         common clocks (both are called recoil escapements, from
         the recoil of the escape wheel at each vibration); the
         cylinder escapement, having an open-sided hollow
         cylinder on the balance arbor to control the escape
         wheel; the duplex escapement, having two sets of teeth
         on the wheel; the lever escapement, which is a kind of
         detached escapement, because the pallets are on a lever
         so arranged that the balance which vibrates it is
         detached during the greater part of its vibration and
         thus swings more freely; the detent escapement, used in
         chronometers; the remontoir escapement, in which the
         escape wheel is driven by an independent spring or
         weight wound up at intervals by the clock train, --
         sometimes used in astronomical clocks. When the shape
         of an escape-wheel tooth is such that it falls dead on
         the pallet without recoil, it forms a deadbeat
         escapement.
         [1913 Webster]

WordNet (r) 3.0 (2006):

escapement
    n 1: mechanical device that regulates movement