poison sumac or any of various poison ivy plants (esp. Rhus toxicodendron and R. diversiloba)
See poison oak in American Heritage Dictionary 4
noun
Either of two shrubs, Toxicodendron toxicarium (syn. Rhus toxicodendron) of the southeast United States or T. diversilobum (syn. R. diversiloba) of western North America, related to poison ivy and causing a rash on contact.