The Collaborative International Dictionary of English v.0.48:

Pacifier \Pac"i*fi`er\, n.
   1. One who or that which pacifies.
      [1913 Webster]

   2. A small device, usually of rubber or plastic containing a
      protruding nipple-shaped portion which a baby may place in
      its mouth and suck on; -- it is used to keep babies
      contented.
      [PJC]

WordNet (r) 3.0 (2006):

pacifier
    n 1: someone who tries to bring peace [syn: conciliator,
         make-peace, pacifier, peacemaker, reconciler]
    2: anything that serves to pacify
    3: device used for an infant to suck or bite on [syn:
       comforter, pacifier, baby's dummy, teething ring]