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 separate eggs that are fertilized by two separate sperms.
Twins in wrestling
Throughout the history of wrestling, many different promotions have brought in many twin wrestlers. This generally becomes the wrestler's gimmick. Generally, twins have an advantage in most matches as they can "switch out" of action without the referee knowing.