occasional cause

Webster's Dictionary (GCIDE)

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]