The Collaborative International Dictionary of English v.0.48:

Involucre \In"vo*lu`cre\ (?; 277), n. [L. involucrum a covering,
   wrapper, fr. involvere to wrap up, envelop: cf. F. involucre.
   See Involve.] (Bot.)
   (a) A whorl or set of bracts around a flower, umbel, or head.
   (b) A continuous marginal covering of sporangia, in certain
       ferns, as in the common brake, or the cup-shaped
       processes of the filmy ferns.
   (c) The peridium or volva of certain fungi. Called also
       involucrum.
       [1913 Webster]

WordNet (r) 3.0 (2006):

involucre
    n 1: a highly conspicuous bract or bract pair or ring of bracts
         at the base of an inflorescence