The Collaborative International Dictionary of English v.0.48:

octagonal \oc*tag"o*nal\, a.
   Having eight sides and eight angles.
   [1913 Webster]

WordNet (r) 3.0 (2006):

octagonal
    adj 1: of or relating to or shaped like an octagon [syn:
           octangular, octagonal]

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "octagonal":
   decagonal, dodecagonal, dodecahedral, eightfold, eighth,
   heptagonal, hexagonal, hexahedral, icosahedral, octadic,
   octahedral, octal, octan, octaploid, octastyle, octofid, octonary,
   octosyllabic, octuple, pentagonal, pentahedral