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.

Source: Webster's Revised Unabridged Dictionary (1913)