poison ivy

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

• noun A North American shrub or vine (Toxicodendron radicans) that has compound leaves with three leaflets, small green flowers, and whitish berries and that causes a rash on contact.
• noun A skin rash caused by contact with this plant.

from The Century Dictionary.

• noun A shrub-vine of North America, Rhus Toxicodendron, sometimes low and erect, but commonly a climber on trees, rocks, fences, etc.

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

• noun A woody vine plant in the family Anacardiaceae well-known for its ability to produce urushiol, a skin irritant that causes an itching rash and blistering for most people.

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 the poison ivy plant