The Collaborative International Dictionary of English v.0.48:

Ratchet \Ratch"et\ (-[e^]t), n. [Properly a diminutive from the
   same word as rack: cf. F. rochet. See 2d Ratch, Rack the
   instrument.]
   1. A pawl, click, or detent, for holding or propelling a
      ratchet wheel, or ratch, etc.
      [1913 Webster]

   2. A mechanism composed of a ratchet wheel, or ratch, and
      pawl. See Ratchet wheel, below, and 2d Ratch.
      [1913 Webster]

   Ratchet brace (Mech.), a boring brace, having a ratchet
      wheel and pawl for rotating the tool by back and forth
      movements of the brace handle.

   Ratchet drill, a portable machine for working a drill by
      hand, consisting of a hand lever carrying at one end a
      drill holder which is revolved by means of a ratchet wheel
      and pawl, by swinging the lever back and forth.

   Ratchet wheel (Mach.), a circular wheel having teeth,
      usually angular, with which a reciprocating pawl engages
      to turn the wheel forward, or a stationary pawl to hold it
      from turning backward.
      [1913 Webster]
      [1913 Webster]

   Note: In the cut, the moving pawl c slides over the teeth in
         one direction, but in returning, draws the wheel with
         it, while the pawl d prevents it from turning in the
         contrary direction.
         [1913 Webster]

WordNet (r) 3.0 (2006):

ratchet wheel
    n 1: toothed wheel held in place by a pawl or detent and turned
         by a lever