Celastrus

Celastrus, commonly known as staff vine, staff tree or bittersweet, is a genus in the family Celastraceae which comprises about 30-40 species of shrubs and vines. They have a wide distribution in East Asia, Australasia, Africa, and the Americas.
The leaves are alternate and simple, ovoid, and typically long. The flowers are small, white, pink or greenish, and borne in long panicles; the fruit is a three-valved berry.
In North America, they are known as bittersweet, presumably a result of confusion with the unrelated bittersweet ("Solanum dulcamara") by early colonists. "C. orbiculatus" is a serious invasive weed in much of eastern North America.