adj. & n.
--adj.
1. lasting two years.
2 recurring every two years (cf. BIANNUAL).
--n.
1. Bot. a plant that takes two years to grow from seed to fruition and die (cf. ANNUAL, PERENNIAL).
2 an event celebrated or taking place every two years.
Derivatives:
biennially adv.
Etymology: L biennis (as BI-, annus year)