The Collaborative International Dictionary of English v.0.48:

Triennial \Tri*en"ni*al\, n.
   Something which takes place or appears once in three years.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. Happening, coming about, or appearing once in every three
      years; as, triennial elections; a triennial catalogue; a
      triennial visitation. --T. Warton.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Cyclic \Cyc"lic\ (s?k"l?k or s?"kl?k), Cyclical \Cyc"lic*al\
   (s?k"l?-kal), a. [Cf. F. cycluque, Gr. kykliko`s, fr. ky`klos
   See Cycle.]
   1. Of or pertaining to a cycle or circle; moving in cycles;
      as, cyclical time. --Coleridge.
      [1913 Webster]

   2. (Chemistry) Having atoms bonded to form a ring structure.
      Opposite of acyclic.

   Note: Used most commonly in respect to organic compounds.

   Note: [Narrower terms: bicyclic; heterocyclic;
         homocyclic, isocyclic]

   Syn: closed-chain, closed-ring.
        [WordNet 1.5]

   3. Recurring in cycles[2]; having a pattern that repeats at
      approximately equal intervals; periodic. Opposite of
      noncyclic.

   Note: [Narrower terms: alternate(prenominal),
         alternating(prenominal); alternate(prenominal), every
         other(prenominal), every second(prenominal);
         alternating(prenominal), oscillating(prenominal);
         biyearly; circadian exhibiting 24-hour
         periodicity); circular; daily, diurnal;
         fortnightly, biweekly; hourly; midweek,
         midweekly; seasonal; semestral, semestrial;
         semiannual, biannual, biyearly; semiweekly,
         biweekly; weekly; annual, yearly; biennial;
         bimonthly, bimestrial; half-hourly; half-yearly;
         monthly; tertian, alternate(prenominal);
         triennial]
         [WordNet 1.5]

   4. Marked by repeated cycles[2].
      [WordNet 1.5]

   Cyclic chorus, the chorus which performed the songs and
      dances of the dithyrambic odes at Athens, dancing round
      the altar of Bacchus in a circle.

   Cyclic poets, certain epic poets who followed Homer, and
      wrote merely on the Trojan war and its heroes; -- so
      called because keeping within the circle of a single
      subject. Also, any series or coterie of poets writing on
      one subject. --Milman.
      [1913 Webster]

WordNet (r) 3.0 (2006):

triennial
    adj 1: occurring every third year or lasting 3 years
    n 1: the 300th anniversary (or the celebration of it) [syn:
         tercentennial, tercentenary, triennial]

Moby Thesaurus II by Grady Ward, 1.0:

89 Moby Thesaurus words for "triennial":
   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