one of a pair of twins who are physically joined and often share some organs. Fusion is typically along the trunk of the body or at the front, side, or back of the head.

In the case of symmetrical conjoined twins, the children usually have no birth anomalies except at the areas of fusion. In cases where each twin has enough tissue and organs for independent survival, they are often successfully separated by surgery. In the case of nonsymmetric conjoined twins, one is fairly well developed, but the other is severely underdeveloped, often tiny, and dependent on the larger twin for nutrition. The underdeveloped twin may be surgically separated from the larger asymmetric twin in order to save the one better able to survive.

Siamese twins, the term formerly used for these children, originally referred to the conjoined twins Chang and Eng, who were born in 1811 to parents in Siam (now Thailand) and became widely known from their tours in the West. Chang and Eng were joined by a ligament from breastbone to navel.