The Collaborative International Dictionary of English v.0.48:

Seasonal \Sea"son*al\, a.
   1. Of or pertaining to the seasons.
      [1913 Webster]

   2. Occurring or being used in a specific season; as, seasonal
      items for sale.
      [1913 Webster]

   Seasonal dimorphism (Zool.), the condition of having two
      distinct varieties which appear at different seasons, as
      certain species of butterflies in which the spring brood
      differs from the summer or autumnal brood.
      [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):

seasonal
    adj 1: occurring at or dependent on a particular season;
           "seasonal labor"; "a seasonal rise in unemployment" [ant:
           year-around, year-round]
    n 1: a worker who finds employment only in certain seasons [syn:
         seasonal worker, seasonal]

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "seasonal":
   aestival, alternate, arctic, autumn, autumnal, beating, boreal,
   brumal, canicular, circling, cyclic, epochal, equinoctial, even,
   every other, hibernal, hiemal, intermittent, isochronal, measured,
   metronomic, midsummer, midwinter, oscillatory, out of season,
   periodical, pulsing, reciprocal, recurrent, recurring, rhythmic,
   rotary, serial, solstitial, spring, springlike, steady, summer,
   summerlike, summerly, summery, undulant, undulatory, vernal,
   wavelike, wheeling, winter, winterlike, wintery, wintry