incidental has 13 distinct synonyms spread across 4 of its 5 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 basic.
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.
not of prime or central importance
“nonessential to the integral meanings of poetry”
occurring with or following as a consequence
“an excessive growth of bureaucracy, with attendant problems”
(frequently plural) an expense not budgeted or not specified
“he requested reimbursement of $7 for incidental expenses”
an item that is incidental
Synonyms, antonyms and the broader and narrower relations for incidental come from Princeton University’s WordNet. Full sources and licences.