mistletoe

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

• noun Any of various semiparasitic plants of the order Santalales that grow on the branches of other plants, especially Viscum album of Eurasia and Phoradendron leucarpum of North America, both of which have leathery evergreen leaves and waxy white berries. Extracts of the Eurasian species are sometimes used for medicinal purposes.
• noun A sprig of mistletoe, often used as a Christmas decoration.

from The Century Dictionary.

• noun A European plant, Viscum album, of the natural order Loranthaceæ, growing parasitically on various trees.
• noun A plant of some other species of Viscum, or of one of the genera Loranthus, Phoradendron, and Arceuthobium, their species almost all having the same parasitic habit.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Bot.) A parasitic evergreen plant of Europe (Viscum album), bearing a glutinous fruit. When found upon the oak, where it is rare, it was an object of superstitious regard among the Druids. A bird lime is prepared from its fruit.

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

• noun Any of several parasitic evergreen plants with white berries that grow in the crowns of oaks, apple trees and other trees.
• noun A sprig of these plants used as a Christmas decoration.

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

• noun shrub of central and southeastern Europe; partially parasitic on beeches, chestnuts and oaks
• noun American plants closely resembling Old World mistletoe
• noun Old World parasitic shrub having branching greenish stems with leathery leaves and waxy white glutinous berries; the traditional mistletoe of Christmas