biennial (adj.) Look up biennial at Dictionary.com
"lasting for two years" (1620s); "occurring every two years" (1750), from Latin biennium "two-year period," from bi- (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" is attested by 1770.