
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