Psilocybin (pronounced /ˌsaɪlɵˈsaɪbɪn/ Sil-lə-SYE-bin) (also known as psilocybine) is a hallucinogenic (entheogenic, psychedelic) indole of the tryptamine family. It is produced by hundreds of species of fungi, including those of the genus Psilocybe, such as Psilocybe cubensis and Psilocybe semilanceata, and has also been reportedly isolated from about a dozen other genera. Collectively known as psilocybin mushrooms, these are commonly called “boomers,” “sacred mushrooms,” “magic mushrooms,” or more simply “shrooms.” Possession, and in some cases usage, of psilocybin or psilocin has been outlawed in most countries across the globe. Proponents of its usage consider it to be an entheogen and a tool to supplement various types of practices for transcendence, including in meditation, psychonautics, and psychedelic psychotherapy. The intensity and duration of entheogenic effects of psilocybin mushrooms are highly variable, depending on species or cultivar of mushrooms, dosage, individual physiology, and set and setting.

Once ingested, psilocybin is rapidly metabolised to psilocin, which then acts as a partial agonist at the 5-HT_2A and 5-HT_1Aserotonin receptors in the brain. The mind-altering effects of psilocybin typically last anywhere from 3 to 8 hours; however, to individuals under the influence of psilocybin, the effects may seem to last much longer, since the drug can distort the perception of time.