Psychoactive substances are substances that, when taken in or administered into one's system, affect mental processes, e.g., cognition or affect.
This term and its equivalent, psychotropic drug, are the most neutral and descriptive term for the whole class of substances, licit and illicit, of interest to drug policy.

"Psychoactive" does not necessarily imply dependence-producing, and in common parlance, the term is often left unstated, as in "drug use" or "substance abuse."

World Health Organization. Terminology and classification. Internet. Accessed November 11, 2009.