(1):

(n.) Something which takes place or appears once in two years; esp. a biennial examination.

(2):

(a.) Happening, or taking place, once in two years; as, a biennial election.

(3):

(a.) Continuing for two years, and then perishing, as plants which form roots and leaves the first year, and produce fruit the second.

(4):

(n.) A plant which exists or lasts for two years.