occurring or appearing at irregular or infrequent intervals; occurring now and then

an occasional headache.

intended for supplementary use when needed

an occasional chair.

pertaining to, arising out of, or intended for the occasion

occasional verses.

acting or serving for the occasion or only on particular occasions.

serving as the occasion or incidental cause.

adjective

constant , frequent , regular , steady , usual

adjective

casual , desultory , especial , exceptional , exclusive , few , incidental , infrequent , intermittent , not habitual , odd , off and on * , particular , random , rare , scarce , seldom , special , specific , uncommon , unfrequent , unusual , fitful , periodic , periodical , sporadic , episodic , irregular , scattered , stray