Twins are two offspring produced by the same pregnancy. Twins can either be monozygotic ("identical"), meaning that they can develop from just one zygote that will then split and form two embryos, or dizygotic ("fraternal"), meaning that they can develop from two different eggs, each are 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.

  
Marian and Vivian Brown, identical twins, 
photographed by Christopher Michel