nonstandard has no recorded synonym. 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 standard.
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 conforming to the language usage of a prestige group within a community
“a nonstandard dialect is one used by uneducated speakers or socially disfavored groups”
varying from or not adhering to a standard
“nonstandard windows”
not standard; not accepted as a model of excellence
“a nonstandard text”
Synonyms, antonyms and the broader and narrower relations for nonstandard come from Princeton University’s WordNet. Full sources and licences.