poison oak

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. 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.
2. n. See poison ivy.
3. n. A skin rash caused by contact with one of these plants.

Century Dictionary and Cyclopedia

1. n. The poison-ivy, or properly its low form; also, the kindred plant of Pacific North America, Rhus diversiloba, which is similarly poisonous and not high-climbing. The latter is also called yeara.

Wiktionary

1. n. Any of several species of Toxicodendron with oak-like leaves.

GNU Webster's 1913

1. n. (Bot.) a dermatitis-producing plant often lumped together with the poison ivy (Toxicodendron radicans) in common terminology, but more properly distinguished as the more shrubby Toxicodendron quercifolium (syn. Toxicodendron diversilobum), common in California and Oregon. Opinion varies as to whether the poison oak and poison ivy are only variants of a single species. See poison ivy, above.

WordNet 3.0

1. n. climbing plant common in eastern and central United States with ternate leaves and greenish flowers followed by white berries; yields an irritating oil that causes a rash on contact
2. n. dermatitis resulting from contact with a poison oak plant