Source : Webster's Revised Unabridged Dictionary (1913)

Sensitizer \Sen"si*ti`zer\, n. (Photog.)
   An agent that sensitizes.

         The sensitizer should be poured on the middle of the
         sheet.                                   --Wilis &
                                                  Clements (The
                                                  Platinotype).

Source : WordNet®

sensitizer
     n : (chemistry) a substance other than a catalyst that
         facilitates the start of a catalytic reaction [syn: {sensitiser}]