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.
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   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]