The Collaborative International Dictionary of English v.0.48:

Whitener \Whit"en*er\, n.
   1. One who, or that which, whitens; a bleaching agent; a
      bleach; a bleacher; a blancher; a whitewasher.
      [1913 Webster]

   2. A chemical used as an adjunct to laundering white cloth,
      which makes white cloth appear whiter. A bluing agent.
      [PJC]

WordNet (r) 3.0 (2006):

whitener
    n 1: an agent that makes things white or colorless [syn:
         bleaching agent, bleach, blanching agent, whitener]