Merriam-Webster Collegiate Dictionary - poison ivy

 noun  Date: 1784  1.  a. a climbing plant (Toxicodendron radicans syn. Rhus radicans) of the cashew family that is especially common in the eastern and central U.S., that has ternate leaves, greenish flowers, and white berries, and that produces an acutely irritating oil causing a usually intensely itching skin rash  b. any of several plants closely related to ~  2. a skin rash caused by ~