Twin

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