Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

heptagon
     n : a seven-sided polygon