The Collaborative International Dictionary of English v.0.48:

Whitecap \White"cap`\, n.
   1. (Zool.)
      (a) The European redstart; -- so called from its white
          forehead.
      (b) The whitethroat; -- so called from its gray head.
      (c) The European tree sparrow.
          [1913 Webster]

   2. A wave whose crest breaks into white foam, as when the
      wind is freshening.
      [1913 Webster]

   3. A member of a self-appointed vigilance committee
      attempting by lynch-law methods to drive away or coerce
      persons obnoxious to it. Some early ones wore white hoods
      or masks. [U. S.] -- White"cap`, v. -- White"cap`per,
      n.
      [Webster 1913 Suppl.]

The Collaborative International Dictionary of English v.0.48:

Whitethroat \White"throat`\, n. (Zool.)
   Any one of several species of Old World warblers, esp. the
   common European species (Sylvia cinerea), called also
   strawsmear, nettlebird, muff, and whitecap, the
   garden whitethroat, or golden warbler (Sylvia
   hortensis), and the lesser whitethroat (Sylvia curruca).
   [1913 Webster]

WordNet (r) 3.0 (2006):

whitecap
    n 1: a wave that is blown by the wind so its crest is broken and
         appears white [syn: whitecap, white horse]