Psychedelics are a subclass of hallucinogenic drugs whose primary effect is to trigger non-ordinary mental states (known as psychedelic experiences or psychedelic "trips") and/or an apparent expansion of consciousness. Sometimes, they are called classic hallucinogens, serotonergic hallucinogens, or serotonergic psychedelics, and the term psychedelic is sometimes used more broadly to include various types of hallucinogens or those which are atypical or adjacent to psychedelia such as MDMA or cannabis; this article uses the narrower definition of psychedelics. True psychedelics cause specific psychological, visual, and auditory changes, and oftentimes a substantially altered state of consciousness. Psychedelic states are often compared to meditative, psychodynamic or transcendental types of alterations of mind. The "classical" psychedelics, the psychedelics with the largest scientific and cultural influence, are mescaline, LSD, psilocybin, and DMT. LSD in particular has long been considered the paradigmatic psychedelic compound, to which all other psychedelics are often or usually compared.

Most psychedelic drugs fall into one of the three families of chemical compounds: tryptamines, phenethylamines, or lysergamides (LSD is considered both a tryptamine and lysergamide). They act via serotonin 2A receptor agonism. When compounds bind to serotonin 5-HT_2A receptors, they modulate the activity of key circuits in the brain involved with sensory perception and cognition, however, the exact nature of how psychedelics induce changes in perception and cognition via the 5-HT_2A receptor is still unknown, although reduction in default mode network activity and increased functional connectivity between regions in the brain as a result may be one of the most relevant pharmacological mechanisms underpinning the psychedelic experience, particularly ego death. The psychedelic experience is often compared to non-ordinary forms of consciousness such as those experienced in meditation, mystical experiences, and near-death experiences, which also appear to be partially underpinned by altered default mode network activity. The phenomenon of ego death is often described as a key feature of the psychedelic experience.

Many psychedelic drugs are illegal worldwide under the UN conventions, with occasional exceptions for religious use or research contexts. Despite these controls, recreational use of psychedelics is common. Legal barriers have made the scientific study of psychedelics more difficult. Research has been conducted, however, and studies show that psychedelics are physiologically safe and rarely lead to addiction. Studies conducted using psilocybin in a psychotherapeutic setting reveal that psychedelic drugs may assist with treating depression, alcohol addiction, and nicotine addiction. Although further research is needed, existing results suggest that psychedelics could be effective treatments for certain forms of psychopathology. A 2022 survey found that 28% of Americans had used a psychedelic at some point in their life.