Source : Webster's Revised Unabridged Dictionary (1913)

Occasional \Oc*ca"sion*al\, a. [Cf.F. occasionnel.]
   1. Of or pertaining to an occasion or to occasions; occuring
      at times, but not constant, regular, or systematic; made
      or happening as opportunity requires or admits; casual;
      incidental; as, occasional remarks, or efforts.

            The . . . occasional writing of the present times.
                                                  --Bagehot.

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

   {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.