Source : Webster's Revised Unabridged Dictionary (1913)

Ratchet \Ratch"et\, 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.

   2. A mechanism composed of a ratchet wheel, or ratch, and
      pawl. See {Ratchet wheel}, below, and 2d {Ratch}.

   {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.

Source : WordNet®

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