A twin is one of two offspring produced in the same pregnancy. Twins can either be identical (in scientific usage, "monozygotic"), meaning that they develop from one zygote that splits and forms two embryos, or fraternal ("dizygotic") because they develop from two separate eggs that are fertilized by two separate sperm.

In contrast, a fetus which develops alone in the womb is called a singleton, and the general term for one offspring of a multiple birth is multiple. It is theoretically possible for two singletons to be identical if all 23 chromosomes in both gametes from the mother and father were to be exact matches from one birth to the next. While this is statistically improbable (less than one in one billion-billion-billion chance) under natural conditions, a controlled pairing may someday be possible. A less complex way to engineer genetically identical offspring is through the process of cloning, a procedure that has successfully been accomplished with several species of mammals.