1. (a.) Happening or appearing once in every three years; as, triennial elections; a triennial visitation\.

2. (a.) Continuing three years; as, triennial parliaments; a triennial reign.

3. (n.) Something which takes place or appears once in three years.

amphibian angiosperm anniversary annual annual holiday aquatic plant biannual bicentenary bicentennial biennial 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 vascular plant vegetable wedding anniversary weed weekly yearly