biennial

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Lasting or living for two years.
• adjective Happening every second year.
• adjective Botany Having a life cycle that normally takes two growing seasons to complete.
• noun An event that occurs every two years.
• noun A plant that normally requires two seasons to complete its life cycle, growing usually as a rosette in the first season and producing flowers and fruits and then dying in the second season.
• noun A perennial plant, such as the English daisy, cultivated as a biennial.

from The Century Dictionary.

• Happening or taking place once in two years: as, biennial games.
• Continuing or lasting for two years; changed or renewed every two years: Said especially of plants.
• noun A plant which requires two seasons of growth to produce its flowers and fruit, growing one year and flowering, fruiting, and dying the next.
• noun An exercise, as a college examination, occurring once in two years. Sometimes also bisannual.

from the GNU version of the Collaborative International Dictionary of English.

• noun Something which takes place or appears once in two years; esp. a biennial examination.
• noun (Bot.) A plant which exists or lasts for two years.
• adjective Happening, or taking place, once in two years.
• adjective (Bot.) Continuing for two years, and then perishing, as plants which form roots and leaves the first year, and produce fruit the second.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective Happening every two years.
• adjective Lasting for two years.
• noun 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.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective occurring every second year
• adjective having a life cycle lasting two seasons
• noun (botany) a plant having a life cycle that normally takes two seasons from germination to death to complete; flowering biennials usually bloom and fruit in the second season