Source : Webster's Revised Unabridged Dictionary (1913)

Tri- \Tri-\ [Gr. tri- or L. tri-, sometimes through French; akin
   to L. tres three, and E. three. See {Three}.]
   1. A prefix meaning three, thrice, threefold; as in
      tricolored, tridentate.

   2. (Chem.) A prefix (also used adjectively) denoting three
      proportional or combining part, or the third degree of
      that to the name of which it is prefixed; as in
      trisulphide, trioxide, trichloride.