conjoined twin

synonym: Siamese twin
https://doi.org/10.1351/goldbook.10555

One of a pair of identical twins fused together with varying degrees of union and of residual duplication of organs.

See also: parasitic twin
Source:
PAC, 2016, 88, 713. (Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)) on page 739 [Terms] [Paper]