The Collaborative International Dictionary of English v.0.48:

Hypanthium \Hy*pan"thi*um\, n.; pl. L. Hypanthia, E.
   Hypanthiums. [NL., fr. Gr. "ypo beneath + 'a`nqos flower.]
   (Bot.)
   A fruit consisting in large part of a receptacle, enlarged
   below the calyx, as in the Calycanthus, the rose hip, and
   the pear.
   [1913 Webster]

WordNet (r) 3.0 (2006):

hypanthium
    n 1: the cuplike or ringlike or tubular structure of a flower
         which bears the sepals and stamens and calyx (as in
         Rosaceae) [syn: hypanthium, floral cup, calyx tube]