[trahy-en-ee-uh l]
  1. a third anniversary.
  2. something that appears or occurs every three years.
  3. a period of three years; triennium.

Origin of triennial

First recorded in 1555–65; trienni(um) + -al1
Related formstri·en·ni·al·ly, adverb Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Examples from the Web for triennially

Historical Examples of triennially

British Dictionary definitions for triennially


  1. relating to, lasting for, or occurring every three years
  1. a third anniversary
  2. a triennial period, thing, or occurrence
Derived Formstriennially, adverb

Word Origin for triennial

C17: from Latin triennium
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 triennially



1630s, "lasting three years;" 1640s, "occurring every three years," from Latin triennium "three-year period," from tri- "three" (see tri-) + annus "year" (see annual). For vowel change, see biennial.

Online Etymology Dictionary, © 2010 Douglas Harper