tri·en·ni·al /(ˌ)traɪˈɛniəl/
  (a.)每三年一次的三年生植物,三年發生一次的事
  Tri·en·ni·al a.
  1. Continuing three years; as, triennial parliaments; a triennial reign.
  2. Happening, coming about, or appearing once in every three years; as, triennial elections; a triennial catalogue; a triennial visitation.
  Tri·en·ni·al, n. Something which takes place or appears once in three years.
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  triennial
       adj : occurring every third year or lasting 3 years
       n : the 300th anniversary (or the celebration of it) [syn: tercentennial,
            tercentenary]