A psychoactive substance, the production, sale, or use of which is prohibited.

Strictly speaking, it is not the drug that is illicit, but its production, sale, or use in particular circumstances in a given jurisdiction (see Controlled substances).

"Illicit drug market," a more exact term, refers to the production, distribution, and sale of any drug outside of legally sanctioned channels.

World Health Organization. Lexicon of alcohol and drug terms published by the World Health Organization. Internet. Accessed August 20, 2009.