occasional

adjective

1. occurring or appearing at irregular or infrequent intervals; occurring now and then: an occasional headache.
2. intended for supplementary use when needed: an occasional chair.
3. pertaining to, arising out of, or intended for the occasion: occasional verses.
4. acting or serving for the occasion or only on particular occasions.
5. serving as the occasion or incidental cause.

adjective

1. taking place from time to time; not frequent or regular
2. of, for, or happening on special occasions
3. serving as an occasion (for something)

adj.late 14c., “occurring now and then,” from occasion (n.) + -al (1). Meaning “casual” is 1560s. Meaning “happening on or pertaining to a particular occasion” is from 1630s. Of furniture, etc., from 1749.