In fiction and folklore, a doppelgänger (German "double walker") (pronounced [ˈdɔpəlˌɡɛŋɐ] ( listen)) is a paranormal double of a living person, typically representing evil or misfortune.
In modern vernacular it is simply any double or look-alike of a
person. It also describes the sensation of having glimpsed oneself in peripheral vision,
in a position where there is no chance that it could have been a
reflection. Doppelgängers often are perceived as a sinister form of bilocation and are regarded by some to be harbingers
of bad luck. In some traditions, a doppelgänger seen by a person's
friends or relatives portends illness or danger, while seeing one's own
doppelgänger is an omen of death.
Source: http://en.wikipedia.org/wiki/Doppelg%C3%A4nger