What is "sexennial"

Sexennial \Sex*en"ni*al\, a. [L. sexennium a period of six years, sexennis of six years; sex six + annus a year. See Six, and Annual.] Lasting six years, or happening once in six years. -- n. A sexennial event.

a. 1 Lasting six years. 2 Happening once in six years. n. A sexennial event.