A twin is generally defined as offspring produced in the same pregnancy and/or born at the same birth process, although people with unusually high genetical similarity also may be called twins although they may not be born at the same time (possibly by Caesarean section or surrogacy), such as with monozygotic twins. Offspring that are not genetically identical but are born at the same time are called dizygotic twins. In contrast, the general term for more than one offspring in the same pregnancy (multiple birth) is multiples; a fetus which develops alone in the womb is called a singleton.
Text source