periodic has 2 distinct synonyms spread across 2 of its 2 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is aperiodic.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
happening or recurring at regular intervals
“the periodic appearance of the seventeen-year locust”
Synonyms, antonyms and the broader and narrower relations for periodic come from Princeton University’s WordNet. Full sources and licences.