trigon

noun

1. (countable, rare)A triangle.
2. (countable, historical)An ancient triangular harp of Oriental origin which had four strings and was often used for banquet music. Also called sabbeka, sackbut, sambuca.
3. (countable)A division consisting of three star signs.

   “The Cancer trigon is composed of Cancer, Scorpio and Pisces”

4. (countable)A trine; an aspect of two planets distant 120 degrees from each other.

   “In the horoscope of Christ's nativity.[…]Whether a trigon or no, this tetragon I am sure there was.”

5. (historical, uncountable)An old ball game played by three people standing in a triangular formation. (See Wikipedia's entry for the game)
6. (countable)The cusp (cutting region) of the crown of an upper molar, usually the anterior part.