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From 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]