The Collaborative International Dictionary of English v.0.48:

Quinquennial \Quin*quen"ni*al\, a. [L. quinquennalis and
   quinquennis; quinque five + annus year. See Five, and cf.
   Biennial.]
   Occurring once in five years, or at the end of every five
   years; also, lasting five years. A quinquennial event.
   [1913 Webster]