biennial

A plant which flowers, sets seed, and dies in its second year. In the first year it produces only vegetative growth, usually a rosette of leaves which build up food reserves for the onset of flowering.

Biennial is a term referring to a period of two years, much in the same way centennial refers to 100 years. If an event occurs every two years, it can be said to occur biennially. These words have become ambiguous, as biannual can mean occurring twice a year or occurring once every two years, and semiannual and biannual share the twice a year definition. In this case, prefixing bi- (meaning "two") to annual can either indicate twice annually, or a period twice the length of "annual".