The Collaborative International Dictionary of English v.0.48:

Oxidizer \Ox"i*di`zer\, n. (Chem.)
   An agent employed in oxidation, or which facilitates or
   brings about combination with oxygen; as, nitric acid,
   chlorine, bromine, etc., are strong oxidizers.
   [1913 Webster]

WordNet (r) 3.0 (2006):

oxidizer
    n 1: a substance that oxidizes another substance [syn:
         oxidant, oxidizer, oxidiser, oxidizing agent]