Why is the ninth month called September?
"lasting for two years" (1620s); "occurring every two years" (1750), from Latin biennium "two-year period," from bi- (see bi-) + annus "year" (see annual). 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" is attested by 1770.
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.