A Bactrician triad is a bonded group of three Bactricians -- a male, a female, and a genderless "Third". All three are necessary in order to complete the act of procreation. A Triad is a lifetime bond, and when the other members of the Triad die, the Third is expected also to take its life. (DS9 - The Lives of Dax short story: "The Music Between the Notes")