See occasional in Webster's New World Roget's A-Z Thesaurus II
adjective
Happening or appearing now and then:
fitful, intermittent, periodic, periodical, sporadic. (Informal) on-again, off-again. See continueRarely occurring or appearing:
infrequent, rare, scarce, sporadic, uncommon, unusual. Idiom: few and far between. See usual
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