(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.