Lewis & Short

pentăgōnĭus, a, um, adj., = πενταγώνιος and pentăgōnus, a, um, = πεντάγωνος, quinquangular, pentagonal.
Subst.: pentăgōnĭum, i, n., a pentagon (post-class.): pentagonii ratio, Auct. de Limit. p. 257 Goes.—pentăgōnum, i, n., for pentaphyllon, App. Herb. 2.