twin

Twins are two babies born at the same time. They can be identical twins (from one egg which split) or fraternal twins (from two eggs).

If there are more than two babies, there can be a mixture of identical and fraternal. For instance, if three babies are born, the triplets could be identical twins and a singleton. Four babies, quadruplets, could be two sets of identical twins or four singletons or....

Twin can also be used as a verb. To make a twin for something so as to have a pair.