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 Unabridged Dictionary, © Random House, Inc. 2019

British Dictionary definitions for tri-



three or thricetriaxial; trigon; trisect
occurring every threetrimonthly

Word Origin for tri-

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

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

Medicine definitions for tri-



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.