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The Collaborative International Dictionary

Tri-

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.

Wiktionary

tri-

Etymology 1 pre. three Etymology 2

pre. (context chemistry English) Used in combination with a known element name (e.g. 'tri-silicon') to designate an unknown element that would reside three periods lower than the known element on a Mendeleev-type periodic table. See http://en.wikipedia.org/wiki/Mendeleev's%20predicted%20elements.

Douglas Harper's Etymology Dictionary

tri-

word-forming element meaning "three, having three, once every three," from Latin tres (neuter tria) or Greek treis, trias "three" (see three).

Longman Dictionary of Contemporary English

tri-

prefix
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