A doppelgänger is the ghostly — in some cases, the physical — double of a living person. The word "doppelgänger" is a loanword from German, in which language it is written (as with any German noun) with an initial capital letter: Doppelgänger. The word derives from Doppel ("double") and Gänger ("goer").

In English, the word is conventionally uncapitalized ("doppelgänger"). It is also common to drop the German diacritic, umlaut, from the letter "ä," writing "doppelganger," although in German the correct spelling without the umlaut would actually be "Doppelgaenger."

In the vernacular, "Doppelgänger" has come to refer to any double or look-alike of a person—most commonly an "evil twin"—or to bilocation. It is an exact replica of a specific human being.

The word is also used to describe a phenomenon whereby one catches his own image out of the corner of his eye. In some traditions, seeing one's own doppelgänger is an omen of death, or a warning of an aproaching danger. A doppelgänger seen by a person's friends or relatives may sometimes bring bad luck, or indicate an approaching health problem.