magic mushroom 
noun informal
A toadstool with hallucinogenic properties, especially the liberty cap and its relatives.
- Genus Psilocybe, family Strophariaceae, class Hymenomycetes: several species.
- He is a level-headed fifty-something, who has never, as far as I know, eaten magic mushrooms or swallowed hallucinogenic drugs.
- The new legislation rules that psilocin, the hallucinogen found in magic mushrooms, is illegal and the mushrooms are considered to be controlled drugs.