restrainer

from WordNet (r) 3.0 (2006)

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: {restrainer},
       {controller}]

from The Collaborative International Dictionary of English v.0.48

Restrainer \Re*strain"er\, n.
   One who, or that which, restrains.
   [1913 Webster]