D defs.my
Entry 3 senses Webster, 1913

Occasional

/(ŏk*kā"zhŭn*al)/ · Oc·ca·sion·al · IPA /əˈkeɪʒ(ə)nəl/
01 a. Occuring at times, but not constant, regular, or systematic; made or happening as opportunity requires or admits; casual; incidental; as, o…
  1. 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. 2.
    Produced by accident; as, the occasional origin of a thing.[Obs.]
  3. 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.