A psychedelic substance is a psychoactive drug whose primary action is to alter cognition and perception, typically by agonising serotonin receptors. Psychedelics are part of a wider class of psychoactive drugs known as hallucinogens, a class that also includes mechanistically unrelated substances such as dissociatives and deliriants. Unlike other drugs such as stimulants and opioids which induce familiar states of consciousness, psychedelics tend to affect the mind in ways that result in the experience being qualitatively different from those of ordinary consciousness. The psychedelic experience is often compared to non-ordinary forms of consciousness such as trance, meditation, yoga, religious ecstasy, dreaming and even near-death experiences. With a few exceptions, most psychedelic drugs fall into one of the three following families of chemical compounds; tryptamines, phenethylamines, and lysergamides.

Many psychedelic drugs are illegal worldwide under the UN conventions unless used in a medical or religious context, such as medical cannabis or ayahuasca. Despite these regulations, recreational use of psychedelics is common.