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]