What do we call twins that develop from a single fertilized egg that splits in two?

The term used to describe twins that develop from a single fertilized egg that splits into two embryos is "Identical (Monozygotic) Twins." This occurs when a single zygote, formed from one egg and one sperm, divides and forms two genetically identical individuals. Because they originated from the same fertilized egg, these twins share the same genetic material, leading to very similar physical characteristics and, in many cases, psychological traits.

In contrast, fraternal twins are the result of two separate eggs being fertilized by two separate sperm, leading to genetically distinct siblings. This explains why they do not have the same level of genetic similarity as identical twins. The other answer choices, such as conjoint twins and half-siblings, refer to entirely different concepts in reproductive biology and familial relations. Conjoint twins, for instance, are twins that are physically connected and typically result from incomplete splitting of the embryo, while half-siblings share only one parent genetically. Therefore, the definition of identical twins aligns perfectly with the concept of monozygotic development.