The Collaborative International Dictionary of English v.0.48:
Pentagonal \Pen*tag"o*nal\, a. [Cf. F. pentagonal, pentagone, L. pentagonus, pentagonius, Gr. ?.] Having five corners or angles. [1913 Webster] Pentagonal dodecahedron. See Dodecahedron, and Pyritohedron. [1913 Webster]