Occasional /(ŏk*kā"zhŭn*al)/

Oc·ca·sion·al

Occasional

a.
  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.
    The . . . occasional writing of the present times.
    — Bagehot.
  2. Produced by accident; as, the occasional origin of a thing. [Obs.]
  3. Of or pertaining to an occasion or to occasions; intended for a specific occasion; for use only when needed, and not regularly.

Phrases & Compounds
Occasional cause
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.