A plant that requires two years to complete its life-cycle, germinating and growing in its first year, then producing its flowers and fruit in its second year, after which it usually dies.

Happening every two years. / Lasting for two years.

Taking place every two years

plural of biennial

The condition of being biennial

A quasiperiodic oscillation of the equatorial zonal wind between easterlies and westerlies in the tropical stratosphere

plural of quasi-biennial oscillation