From Longman Dictionary of Contemporary EnglishRelated topics:Chronologytri‧en‧ni‧al/traɪˈeniəl/adjectivehappening every three years→annualExamples from the Corpustriennial•At theirtriennialcongresson Sept. 15-21,1990,delegatesadopteda new partyprogrammereplacingthat of 1975.•June30 ...