Do you really want to send Webster 1913 a message saying you like their work?
Dec"a*gon (?), n. [Pref. deca- + Gr. a corner or angle: cf. F. d'ecagone.] Geom.
A plane figure having ten sides and ten angles; any figure having ten angles. A regular decagon is one that has all its sides and angles equal.
© Webster 1913.