The Collaborative International Dictionary of English v.0.48:

Teasel \Tea"sel\, n. [OE. tesel, AS. t[=ae]sel, t[=ae]sl, the
   fuller's herb. See Tease.] [Written also tassel, tazel,
   teasle, teazel, and teazle.]
   1. (Bot.) A plant of the genus Dipsacus, of which one
      species (Dipsacus fullonum) bears a large flower head
      covered with stiff, prickly, hooked bracts. This flower
      head, when dried, is used for raising a nap on woolen
      cloth.
      [1913 Webster]

   Note: Small teasel is Dipsacus pilosus, wild teasel is
         Dipsacus sylvestris.
         [1913 Webster]

   2. A bur of this plant.
      [1913 Webster]

   3. Any contrivance intended as a substitute for teasels in
      dressing cloth.
      [1913 Webster]

   Teasel frame, a frame or set of iron bars in which teasel
      heads are fixed for raising the nap on woolen cloth.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Dipsacus sylvestris
    n 1: European teasel with white to pink flowers; naturalized in
         United States [syn: wild teasel, Dipsacus sylvestris]