Source : Webster's Revised Unabridged Dictionary (1913)

Triennial \Tri*en"ni*al\, a. [L. triennium the space of three
   years; tri- (see {Tri-}) + annus year. See {Annual}.]
   1. Continuing three years; as, triennial parliaments; a
      triennial reign. --Howell.

   2. Happening, coming about, or appearing once in every three
      years; as, triennial elections; a triennial catalogue; a
      triennial visitation. --T. Warton.

Triennial \Tri*en"ni*al\, n.
   Something which takes place or appears once in three years.

Source : WordNet®

triennial
     adj : occurring every third year or lasting 3 years
     n : the 300th anniversary (or the celebration of it) [syn: {tercentennial},
          {tercentenary}]