adjective (a)

• not conforming to the language usage of a prestige group within a community(adj.all)

  Example:
  A nonstandard dialect is one used by uneducated speakers or socially disfavored groups.
  The common core of nonstandard words and phrases in folk speech.
  

  Antonym:
  received, standard
  

  source: wordnet30
• varying from or not adhering to a standard(adj.all)

  Example:
  Nonstandard windows.
  Envelopes of nonstandard sizes.
  Nonstandard lengths of board.
  

  source: wordnet30
• not standard; not accepted as a model of excellence(adj.all)

  Example:
  A nonstandard text.
  

  Antonym:
  standard
  

  source: wordnet30