Psilocybe semilanceata, commonly known as the liberty cap, is a species of fungus which produces the psychoactive compounds psilocybin, psilocin and baeocystin. It is both one of the most widely distributed psilocybin mushrooms in nature, and one of the most potent. The mushrooms have a distinctive conical to bell-shaped cap, up to 2.5 cm (1 in) in diameter, with a small nipple-like protrusion on the top. They are yellow to brown, covered with radial grooves when moist, and fade to a lighter color as they mature. Their stipes tend to be slender and long, and the same color or slightly lighter than the cap. The gill attachment to the stipe is adnexed, and they are initially cream-colored before tinting purple to black as the spores mature. The spores are dark purplish-brown in mass, ellipsoid in shape, and measure 10.5–15 by 6.5–8.5 micrometres.

Psilocybin mushrooms, commonly known as magic mushrooms or shrooms, are a polyphyletic informal group of fungi that contain psilocybin, which turns into psilocin upon ingestion. Biological genera containing psilocybin mushrooms include Psilocybe, Panaeolus, Inocybe, Pluteus, Gymnopilus, and Pholiotina.