The Collaborative International Dictionary of English v.0.48:

Pentahedron \Pen`ta*he"dron\, n. [Penta- + Gr. "e`dra seat,
   base.]
   A solid figure having five sides.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pentahedron
    n 1: any polyhedron having five plane faces