Wordnet 3.0
NOUN (1)
1.
a period of five years;
The Collaborative International Dictionary of English v.0.48:
Quinquennium \Quin*quen"ni*um\, n. [L.]
Space of five years.
[1913 Webster]
WordNet (r) 3.0 (2006):
quinquennium
n 1: a period of five years