The Collaborative International Dictionary of English v.0.48:

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 I Ching (Yih King), or "Book of
       Changes." --S. W. Williams.
       [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

hexagram
    n 1: a regular polygon formed by extending each of the sides of
         a regular hexagon to form two equilateral triangles