Triennial /(?)/

Tri·en·ni·al

Triennial

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.

Triennial

n.
  1. Something which takes place or appears once in three years.