How do you spell Biennial

Available Definitions:
1)  a. - Happening, or taking place, once in two years; as, a biennial election.
2)  a. - Continuing for two years, and then perishing, as plants which form roots and leaves the first year, and produce fruit the second.
3)  n. - Something which takes place or appears once in two years; esp. a biennial examination.
4)  n. - A plant which exists or lasts for two years.