OCCASIONAL
periodic, occasional
(adjective) recurring or reappearing from time to time; “periodic feelings of anxiety”
occasional
(adjective) occurring from time to time; “took an occasional glass of wine”
episodic, occasional
(adjective) occurring or appearing at usually irregular intervals; “episodic in his affections”; “occasional headaches”
casual, occasional
(adjective) occurring on a temporary or irregular basis; “casual employment”; “a casual correspondence with a former teacher”; “an occasional worker”
Source: WordNet® 3.1
Etymology
Adjective
occasional (not comparable)
Occurring or appearing irregularly from time to time, but not often.
Created for a specific occasion.
Intended for use as the occasion requires.
Acting in the indicated role from time to time.
Noun
occasional (plural occasionals)
A person who does something only occasionally.
Source: Wiktionary
Oc*ca"sion*al, a. Etym: [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.
Source: Webster’s Unabridged Dictionary 1913 Edition