twin [countable]

one of two children born at the same time to the same mother:

The twins are now eight months old.

Siamese twin [countable usually plural]

one of two people who are born joined to each other - sometimes considered offensive [= conjoined twin]

identical twin [countable usually plural]

one of a pair of brothers or sisters born at the same time, who develop from the same egg and look almost exactly alike [↪ opposite of Fraternal twin]