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biennial (adj.)

1620s, "lasting for two years;" 1750, "occurring every two years," from Latin biennium "two-year period," from bi- "two" (see bi-) + annus "year" (see annual (adj.)). The vowel change is "due to the Latin phonetic law according to which the unaccented and closed radical syllable of the second element of compounds, original -ă- becomes -ĕ-" [Klein]. The noun meaning "a biennial plant" (which requires two seasons of growth to produce flowers and fruit and dies the next) is attested by 1770. Related: Biennially.

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Definitions of biennial from WordNet
1
biennial (adj.)
having a life cycle lasting two seasons;
parsnips and carrots are biennial plants often grown as annuals
a biennial life cycle
Synonyms: two-year
biennial (adj.)
occurring every second year;
they met at biennial conventions
Synonyms: biyearly
2
biennial (n.)
(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;
From wordnet.princeton.edu