Before establishing the biological origin differences between identical and fraternal twins, let’s make a quick note on the etymology of both words.

Etymologically both terms have the same origin (Latin gemellicium) and were used interchangeably to refer to children born in the same delivery. The only difference was the use of the term fraternal twin in the popular speech twin and identical twin in the cultivated speech. However, the semantic evolution of both words has been accompanied by the numerous advances in the field of genetics and knowledge of the development of the human embryo. In fact, since they differ in the latest edition of the dictionary of the RAE:

Definition of fraternal and identical twin