The Collaborative International Dictionary of English v.0.48:

Egret \E"gret\, n. [See Aigret, Heron.]
   1. (Zo["o]l.) The name of several species of herons which
      bear plumes on the back. They are generally white. Among
      the best known species are the American egret (Ardea
      egretta syn. Herodias egretta); the great egret (Ardea
      alba); the little egret (Ardea garzetta), of Europe;
      and the American snowy egret (Ardea candidissima).
      [1913 Webster]

            A bunch of egrets killed for their plumage. --G. W.
                                                  Cable.
      [1913 Webster]

   2. A plume or tuft of feathers worn as a part of a headdress,
      or anything imitating such an ornament; an aigrette.
      [1913 Webster]

   3. (Bot.) The flying feathery or hairy crown of seeds or
      achenes, as the down of the thistle.
      [1913 Webster]

   4. (Zo["o]l.) A kind of ape.
      [1913 Webster]

WordNet (r) 3.0 (2006):

egret
    n 1: any of various usually white herons having long plumes
         during breeding season