Real Dictionary

poison ivy
noun: any of several North American anacardiaceous shrubs or vines of the genus Rhus (or Toxicodendron), esp R. radicans, which has small green flowers and whitish berries that cause an itching rash on contact
uncountable noun: Poison ivy is a wild plant that grows in North America and that causes a rash or skin problems if you touch it.

poison ivy in American English
any of several woody vines or shrubs (genus Rhus, esp. R. radicans), of the cashew family, having leaves of three leaflets, greenish flowers, and ivory-colored berries: it can cause a severe rash on contact
noun: a vine or shrub, Rhus radicans, having trifoliate leaves and whitish berries and causing severe dermatitis when touched by persons sensitive to it