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a combining form meaning “three,” used in the formation of compound words:
triacid; triatomic.
Origin of tri-
Middle English < Latin, combining form representing Latin trēs, tria, Greek treîs, tría three Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2018.
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British Dictionary definitions for tri-


three or thrice: triaxial, trigon, trisect
occurring every three: trimonthly
Word Origin
from Latin trēs, Greek treis
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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Word Origin and History for tri-

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

Online Etymology Dictionary, © 2010 Douglas Harper
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tri- in Medicine

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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