WHO – World Health Organization
"A term of varied usage. In medicine, it refers to any substance with the potential to prevent or cure disease or enhance physical or mental welfare, and in pharmacology to any chemical agent that alters the biochemical or physiological processes of tissues or organisms. Hence, a drug is a substance that is, or could be, listed in a pharmacopoeia. In common usage, the term often refers specifically to psychoactive drugs, and often, even more specifically, to illicit drugs, of which there is non-medical use in addition to any medical use."
DSM 5 – Diagnostic and Statistical Manual of Mental Disorders 5th Edition
" 'Psychoactive' (also called "psychotropic") is a term that is applied to chemical substances that change a person's mental state by affecting the way the brain and nervous system work. (This can lead to intoxication, which is often the main reason people choose to take psychoactive drugs.) The changes in brain function experienced by people who use psychoactive substances affect their perceptions, moods, and/or consciousness."
NIDA – National Institute on Drug Abuse
"Drugs act on the central nervous system and alter its normal, everyday activity, causing changes in mood, awareness, and behavior. Psychoactive drugs disrupt the communication between neurons (brain cells), so abusing them can have serious short- and long-term effects on the brain."
MILDT – Mission Interministérielle de Lutte contre la Drogue et la Toxicomanie
(Mission Interministérielle de Lutte contre la Drogue et la Toxicomanie)
"Une drogue est un produit naturel ou synthétique, dont l'usage peut être légal ou non, consommé en vue de modifier l'état de conscience et ayant potentiel d'usage nocif et de dépendance."