bi·en·ni·al 
(bī-ĕnē-əl)
(bī-ĕnē-əl)adj.
1. Lasting or living for two years.
2. Happening every second year.
3. Botany Having a life cycle that normally takes two growing seasons to complete.
n.
1. An event that occurs every two years.
2. Botany
a. A plant that normally requires two seasons to complete its life cycle, growing usually as a rosette in the first season and producing flowers and fruits and then dying in the second season.
b. A perennial plant, such as the English daisy, cultivated as a biennial.
bi·enni·al·ly adv.