Webster's Revised Unabridged Dictionary · 1913

varying from or not adhering to a standard; as, nonstandard windows; envelopes of nonstandard sizes; non-standard lengths of board cost more per foot. Opposite of standard.

(Linguistics) Not conforming to the language usage of a prestige group within a community; as, a nonstandard dialect is one used by uneducated speakers or socially disfavored groups. Opposite of standard.