Siamese twin: Identical (monozygotic) twins that did not separate fully from one another but are still partially united. Due to the incomplete division of one fertilized ovum. Such twins are known medically as conjoined twins.
Conjoined twins are popularly known as Siamese twins after Chang and Eng (1811-1874), the celebrated conjoined Chinese twins born in Siam (Thailand).
Conjoin came via the French from the Latin conjungere, from com- + jungere, to join = to join or yoke together.