English to English

adjective (a)

• prohibited by law or by official or accepted rules(adj.all)

  Example:
  An illegal chess move.
  

  Antonym:
  legal
  

  source: wordnet30
• Not according to, or authorized by, law; specif., contrary to, or in violation of, human law; unlawful; illicit; hence, immoral; as, an illegal act; illegal trade; illegal love.(adjective)

  source: webster1913