The Collaborative International Dictionary of English v.0.48:

Trigon \Tri"gon\, n. [L. trigonum, Gr. ?; ? (see Tri-) + ? a
   corner, angle: cf. F. trigone.]
   [1913 Webster]
   1. A figure having three angles; a triangle.
      [1913 Webster]

   2. (Astrol.)
      (a) A division consisting of three signs.
      (b) Trine, an aspect of two planets distant 120 degrees
          from each other. --Hutton.
          [1913 Webster]

   3. (Gr. & Rom. Antiq.)
      (a) A kind of triangular lyre or harp.
      (b) A kind of game at ball played by three persons
          standing at the angular points of a triangle.
          [1913 Webster]

   4. (Zool.) The cutting region of the crown of an upper molar,
      usually the anterior part. That of a lower molar is the

WordNet (r) 3.0 (2006):

trigon
    n 1: a three-sided polygon [syn: triangle, trigon,
         trilateral]
    2: (astrology) one of four groups of the zodiac where each group
       consists of three signs separated from each other by 120
       degrees [syn: triplicity, trigon]
    3: a triangular lyre of ancient Greece and Rome