The Collaborative International Dictionary of English v.0.48:

Occasional \Oc*ca"sion*al\ ([o^]k*k[=a]"zh[u^]n*al), a. [Cf. F.
   occasionnel.]
   1. Occuring at times, but not constant, regular, or
      systematic; made or happening as opportunity requires or
      admits; casual; incidental; as, occasional remarks, or
      efforts.
      [1913 Webster]

            The . . . occasional writing of the present times.
                                                  --Bagehot.
      [1913 Webster]

   2. Produced by accident; as, the occasional origin of a
      thing. [Obs.] --Sir T. Browne.
      [1913 Webster]

   3. Of or pertaining to an occasion or to occasions; intended
      for a specific occasion; for use only when needed, and not
      regularly.
      [PJC]

   Occasional cause (Metaph.), some circumstance preceding an
      effect which, without being the real cause, becomes the
      occasion of the action of the efficient cause; thus, the
      act of touching gunpowder with fire is the occasional, but
      not the efficient, cause of an explosion.
      [1913 Webster]

WordNet (r) 3.0 (2006):

occasional
    adj 1: occurring from time to time; "took an occasional glass of
           wine"
    2: occurring or appearing at usually irregular intervals;
       "episodic in his affections"; "occasional headaches" [syn:
       episodic, occasional]
    3: occurring from time to time; "casual employment"; "a casual
       correspondence with a former teacher"; "an occasional worker"
       [syn: casual, occasional]
    4: recurring or reappearing from time to time; "periodic
       feelings of anxiety" [syn: periodic, occasional]

Moby Thesaurus II by Grady Ward, 1.0:

74 Moby Thesaurus words for "occasional":
   accessory, accidental, accompanying, additional, ado, adventitious,
   afloat, afoot, aleatory, auxiliary, behind the scenes, by-the-way,
   casual, causal, causative, ceremonial, circumstantial, conditional,
   constitutive, contingent, current, decisive, determinative, doing,
   effectual, etiological, eventuating, extra, few, formative,
   going on, happening, in hand, in the wind, incidental, inessential,
   infrequent, institutive, intermittent, irregular, nonessential,
   occurring, odd, off, on, on foot, ongoing, originative,
   out-of-the-way, parenthetical, part-time, particular, passing,
   periodic, pivotal, prevailing, prevalent, provisional, random,
   rare, resultant, ritual, scarce, seldom, side, spare, special,
   sporadic, supplementary, taking place, uncommon, under way,
   unessential, unfrequent



The Devil's Dictionary (1881-1906):

OCCASIONAL, adj.  Afflicting us with greater or less frequency.  That,
however, is not the sense in which the word is used in the phrase
"occasional verses," which are verses written for an "occasion," such
as an anniversary, a celebration or other event.  True, they afflict
us a little worse than other sorts of verse, but their name has no
reference to irregular recurrence.