1. A plant which lasts but two years.

2. Lasting for only two years; happening once in two years.

Nuttall's Standard dictionary of the English language By Nuttall, P.Austin

• bienniel,
• biennual,
• bienial,
• dennial,
• biennal,
• biennale,
• biannual,
• viennial,
• niennial,
• hiennial,
• binenial,
• bignoniae,
• bigonniad,
• biznoal,
• binial,
• pennial,
• ocennial,
• splennial,
• bianial,
• binennial

• biannually,
• biennially,
• biannual.

The adjective form of the word "biennial" is "biennial".

The plural of the "biennial" can be the "biennials".

The singular of the "biennial" can be the "biennial".