- triennial (adj.)
[triennial 词源字典]
- 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). For vowel change, see biennial. As a noun, 1630s. Related: Triennially.[triennial etymology, triennial origin, 英语词源]