Etymology
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triennial (adj.)

1630s, "lasting three years;" 1640s, "occurring every three years," with -al (1) + Latin triennium "three-year period," from tri- "three" (see tri-) + annus "year" (see annual (adj.)). For vowel change, see biennial. As a noun, 1630s. Related: Triennially.

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Definitions of triennial
1
triennial (n.)
the 300th anniversary (or the celebration of it);
Synonyms: tercentennial / tercentenary
2
triennial (adj.)
occurring every third year or lasting 3 years;
From wordnet.princeton.edu