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]

The Collaborative International Dictionary of English v.0.48:

Whiten \Whit"en\, v. t.
   To make white; to bleach; to blanch; to whitewash; as, to
   whiten a wall; to whiten cloth.
   [1913 Webster]

         The broad stream of the Foyle then whitened by vast
         flocks of wild swans.                    --Macaulay.
   [1913 Webster]

   Syn: See Blanch.
        [1913 Webster]

WordNet (r) 3.0 (2006):

whiten
    v 1: turn white; "This detergent will whiten your laundry" [syn:
         whiten, white] [ant: black, blacken, melanise,
         melanize, nigrify]

Moby Thesaurus II by Grady Ward, 1.0:

79 Moby Thesaurus words for "whiten":
   achromatize, besnow, blanch, bleach, bleach out, blench, blush,
   bowdlerize, chalk, change color, clean, clean out, clean up,
   cleanse, clear out, color, crimson, darken, decolor, decolorize,
   delouse, depurate, deterge, dim, discolor, drain, drain of color,
   dry-clean, dull, dust, dust off, etiolate, expurgate, extenuate,
   fade, fade out, flush, freshen, frost, fume, gloss over, glow,
   grizzle, grow pale, lighten, look black, lose color, lustrate,
   mantle, pale, peroxide, purge, purify, redden, reform, scavenge,
   silver, spruce, steam-clean, sugarcoat, sweep out, sweeten,
   tarnish, tidy, tone down, turn color, turn pale, turn red,
   turn white, varnish, veneer, wan, wash out, white, whitewash, wipe,
   wipe off, wipe out, wipe up