Addiction is a neuropsychological disorder characterized by a persistent and intense urge to use a drug or engage in a behaviour that produces natural reward, despite substantial harm and other negative consequences. Repetitive drug use often alters brain function in ways that perpetuate craving, and weakens self-control. This phenomenon – drugs reshaping brain function – has led to an understanding of addiction as a brain disorder with a complex variety of psychosocial as well as neurobiological factors that are implicated in addiction's development. Classic signs of addiction include compulsive engagement in rewarding stimuli, preoccupation with substances or behavior, and continued use despite negative consequences. Habits and patterns associated with addiction are typically characterized by immediate gratification, coupled with delayed deleterious effects.

व्यसन या आसक्ति (addiction) की विशेषता है कि दुष्परिणामों के बावजूद व्यक्ति को ड्रग/अल्कोहल की बाध्यकारी लत लग जाती है। व्यसन को एक जीर्ण मानसिक रोग भी कह सकते हैं।