a combining form meaning “three,” used in the formation of compound words: triacid; triatomic.

Middle English < Latin, combining form representing Latin trēs, tria, Greek treîs, tría three Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
1.  three or thrice: triaxial; trigon; trisect
2.  occurring every three: trimonthly
[from Latin trēs, Greek treis]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

comb. form meaning "three," from L. tres (neut. tria) or Gk. treis (neut. tria) "three" (see three).
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

tri- pref.

  1. Three: tricuspid.

  2. Containing three atoms, molecules, or groups: triacetic acid.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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