adjective Also, biyearly (for defs 1, 2).
happening every two years: biennial games.
lasting or enduring for two years: a biennial life cycle.
Botany. completing its normal term of life in two years, flowering and fruiting the second year, as beets or winter wheat.
any event occurring once in two years.
Botany. a biennial plant.

1615–25; bienni(um) + -al1

biennially, adverb

biannual, biennial, semiannual (see usage note at bi-).

See bi-1. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
biennial (baɪˈɛnɪəl)
1.  occurring every two years
2.  Compare biannual lasting two years
3.  annual Compare perennial a plant, such as the carrot, that completes its life cycle within two years, developing vegetative storage parts during the first year and flowering and fruiting in its second year
4.  an event that takes place every two years

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

1620s, from L. biennium "two-year period," from bi- + annus "year." The noun meaning "a biennial plant" is attested by 1770.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Science Dictionary
biennial  [%PREMIUM_LINK%]     (bī-ěn'ē-əl)  Pronunciation Key 
Adjective   Completing a life cycle normally in two growing seasons.

Noun   A biennial plant. In the first year, biennials normally produce a short stem, a rosette of leaves, and a fleshy root that acts as food supply. In the second season, biennials blossom, produce seed, use up their food supply, and die. Carrots, parsnips, and sugar beets are examples of biennials. Compare annual, perennial.

The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
Audits conducted biennially under the provisions of this subpart shall cover both years within the biennial period.
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