The Collaborative International Dictionary of English v.0.48:
Heptagon \Hep"ta*gon\, n. [Gr. ? sevencornered; "epta` seven + ? angle: cf. F. heptagone.] (Geom.) A plane figure consisting of seven sides and having seven angles. [1913 Webster]WordNet (r) 3.0 (2006):
heptagon n 1: a seven-sided polygon