The Collaborative International Dictionary of English v.0.48:

Spathe \Spathe\, n. [L. spatha, Gr. ?: cf. F. spathe. See
   Spade for digging.] (Bot.)
   A special involucre formed of one leaf and inclosing a
   spadix, as in aroid plants and palms. See the Note under
   Bract, and Illust. of Spadix.
   [1913 Webster]

   Note: The name is also given to the several-leaved involucre
         of the iris and other similar plants.
         [1913 Webster]

WordNet (r) 3.0 (2006):

spathe
    n 1: a conspicuous bract surrounding or subtending a spadix or
         other inflorescence