Detent

from WordNet (r) 3.0 (2006)

detent
    n 1: a hinged catch that fits into a notch of a ratchet to move
         a wheel forward or prevent it from moving backward [syn:
         {pawl}, {detent}, {click}, {dog}]

from The Collaborative International Dictionary of English v.0.48

Detent \De*tent"\, n. [F. d['e]tente, fr. d['e]tendre to unbend,
   relax; pref. d['e]- (L. dis- or de) + tendre to stretch. See
   {Distend}.] (Mech.)
   That which locks or unlocks a movement; a catch, pawl, or
   dog; especially, in clockwork, the catch which locks and
   unlocks the wheelwork in striking.
   [1913 Webster]