Source : Webster's Revised Unabridged Dictionary (1913)

Hexagram \Hex"a*gram\, n. [Hexa- + -gram.]
   A figure of six lines; specif.:
   (a) A figure composed of two equal triangles intersecting so
       that each side of one triangle is parallel to a side of
       the other, and the six points coincide with those of a
       hexagon.
   (b) In Chinese literature, one of the sixty-four figures
       formed of six parallel lines (continuous or broken),
       forming the basis of the Yih King, or ``Book of
       Changes.''

Source : WordNet®

hexagram
     n : a regular polygon formed by extending each of the sides of a
         regular hexagon to form two equilateral triangles