poison sumac or any of various poison ivy plants (esp. Rhus toxicodendron and R. diversiloba)
See poison oak in American Heritage Dictionary 4
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.