Pentagon (?), n. [Gr. pentagwnon; penta- (see Penta-) + gwnia angle: cf. L. pentagonium, F. pentagone.] (Geom.) A plane figure having five angles, and, consequently, five sides; any figure having five angles.
[1913 Webster]

Regular pentagon, a pentagon in which the angles are all equal, and the sides all equal.
[1913 Webster]