A drug, commonly called a narcotic, is any substance (usually illegal or at least controlled) that causes a physiological change in the body.
Drugs are rather common in the galaxy, usually in shadier circles, and are often used for altering perception, mood, physical abilities, or consciousness. Different substances affect different species, and sometimes a substance that is inert for one species will get another high. Most drugs cause addiction and have varying side effects. Riesel is an example of a planet that sees heavy drug use by its citizens.
Electronic devices can be used to stimulate the senses in a way that mimics drug use, often in sensory lounges and other facilities devoted to such recreation. They can also be used to break an addiction to real drugs.