Educational Content: This article is for informational purposes only and does not constitute medical advice. Always consult a healthcare provider for diagnosis and treatment.
Definition of Siamese twin
Medically reviewed by Dr. Otari Nergadze, Neurosurgeon | Updated: January 2026
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.