Source : Webster's Revised Unabridged Dictionary (1913)

Restrainer \Re*strain"er\, n.
   One who, or that which, restrains.

Source : WordNet®

restrainer
     n 1: a chemical that is added to a photographic developer in
          order to retard development and reduce the amount of fog
          on a film
     2: a person who directs and restrains [syn: {controller}]