Happening every two years. / Lasting for two years.

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.

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