poison oak

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Either of two plants, Toxicodendron pubescens, a shrub of the southeast United States, or T. diversilobum, a shrub or vine of western North America, having compound leaves with three lobed or toothed leaflets, and causing a rash on contact.
• noun A skin rash caused by contact with one of these plants.

from The Century Dictionary.

• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any of several species of Toxicodendron with oak-like leaves.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun 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
• noun dermatitis resulting from contact with a poison oak plant