1. (a.) Happening once in two years; as, a biennial election\.

2. (a.) Continuing for two years, and then perishing, as plants which form roots and leaves the first year, and produce fruit the second.

3. (n.) Something which takes place or appears once in two years; esp. a biennial examination.

4. (n.) A plant which exists or lasts for two years.

amphibian angiosperm anniversary annual annual holiday aquatic plant biannual bicentenary bicentennial bimonthly birthday bissextile day biweekly catamenial centenary centennial commemoration cosmopolite cutting daily decennial deciduous plant diamond jubilee dicot dicotyledon diurnal ephemeral evergreen exotic flowering plant fortnightly fungus gametophyte golden wedding anniversary gymnosperm hebdomadal holy days hourly hydrophyte immovable feast jubilee leap year menstrual momentary momently monocot monocotyl monthly name day natal day octennial perennial plant polycot polycotyl polycotyledon quadrennial quarterly quasquicentennial quincentenary quincentennial quinquennial quotidian secular seed plant seedling semestral semiannual semimonthly semiweekly semiyearly septennial sesquicentennial sextennial silver wedding anniversary spermatophyte sporophyte tercentenary tercentennial tertian thallophyte tricennial triennial vascular plant vegetable wedding anniversary weed weekly yearly