pl. n.
two individuals who are born at the same time and of the same parents. Fraternal (or dizygotic) twins are the result of the simultaneous fertilization of two egg cells; they may be of different sexes and are no more alike than ordinary siblings.
Identical (or monozygotic) twins result from the fertilization of a single egg cell that subsequently divides to give two separate fetuses. They are of the same sex and otherwise genetically identical; any differences in their appearance are due to environmental influences. See also Siamese twins.Medical dictionary. 2011.