poison ivy

American Heritage® Dictionary of the English Language, Fourth Edition

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

Century Dictionary and Cyclopedia

1. n. A shrub-vine of North America, Rhus Toxicodendron, sometimes low and erect, but commonly a climber on trees, rocks, fences, etc. It poisons many persons either by contact or by its effiuvium, causing a severe cutaneous eruption with intense smarting and itching. It is popularly distinguished as three-leafed ivy from the innocuous Virginia creeper, Ampelopsis quinquefolia, the five-leafed ivy, their leaves having respectively three and live leaflets. It is often confounded with the common clematis (Clematis Virginiana), but the trifoliate leaves of that plant are opposite, not alternate as in the poison-ivy. See poison-oak.

Wiktionary

1. n. 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.

GNU Webster's 1913

1. n. (Bot.) an American woody creeper (Rhus Toxicodendron), with trifoliate leaves, and greenish-white berries. It is exceedingly poisonous to the touch for most persons.
2. n. (Bot.) a poisonous climbing plant (formerly Rhus Toxicodendron, or Rhus radicans, now classified as Toxicodendron radicans) of North America. It is common as a climbing vine, especially found on tree trunks, or walls, or as a low, spreading vine or as a shrub. As a low vine it grows well in lightly shaded areas, recognizable by growing in clusters of three leaves. Its leaves are trifoliate, rhombic-ovate, and variously notched. Its form varies slightly from location to location, leading to some speculation that it may consist of more than one species. Many people are poisoned by it, though some appear resistant to its effects. Touching the leaves may leave a residue of an oil on the skin, and if not washed off quickly, sensitive areas of skin become reddened and develop multiple small blisters, lasting for several days to several weeks, and causing a persistent itch. The toxic reaction is due to an oil, present in all parts of the plant except the pollen, called urushiol, the active component of which is the compound pentadecylacatechol (according to Charles H. Booras). See Poison sumac. It is related to poison oak, and is also called mercury.

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 the poison ivy plant