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.