The Collaborative International Dictionary of English v.0.48:

Whitening \Whit"en*ing\, n.
   1. The act or process of making or becoming white.
      [1913 Webster]

   2. That which is used to render white; whiting. [R.]
      [1913 Webster]

   Whitening stone, a sharpening and polishing stone used by
      cutlers; also, a finishing grindstone of fine texture.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Whiten \Whit"en\, v. i. [imp. & p. p. Whitened; p. pr. & vb.
   n. Whitening.] [OE. whitenen; cf. Icel. hv[imac]tna.]
   To grow white; to turn or become white or whiter; as, the
   hair whitens with age; the sea whitens with foam; the trees
   in spring whiten with blossoms.
   [1913 Webster]

WordNet (r) 3.0 (2006):

whitening
    n 1: changing to a lighter color [syn: whitening,
         lightening]

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "whitening":
   achromatization, albification, blanching, bleach, bleaching,
   calcimine, decoloration, decolorization, decolorizing,
   discoloration, etiolation, fading, frosting, grizzling, lightening,
   madder bleach, market bleach, paling, pipe clay, silvering,
   whitewash, whitewashing, whiting