Ersatz means 'substituting for, and typically inferior in quality to', e.g. 'chicory is ersatz coffee'. It is a German word literally meaning substitute or replacement. Although it is used as an adjective in English, Ersatz can only function in German as a noun on its own, or as a part in compound nouns such as Ersatzteile (spare parts) or Ersatzspieler (substitute player). While the English term often implies that the substitution is of unsatisfactory or inferior quality ("not as good as the real thing"), it does not have this connotation in German. The German word for such product is Surrogat (surrogate).

In English, "ersatz" arose as a pejorative during World War II because Ersatzbrot (replacement bread) was given frequently to the POWs, which was made of the lowest-grade flour, potato starch and frequently intermixed with other extenders such as sawdust. This practice was prevalent on the Eastern front and at the many labor and death camps organized by the Nazi regime.

In Britain, this was additionally popularized as an adjective, from the experiences of thousands of U.S., British, and other English-speaking combat personnel, primarily airmen, who were captured in the European Theater of Operations during World War II. These Allied Kriegsgefangene (prisoners of war) were served Ersatzkaffee (a coffee substitute) by their German captors. This substitute drink (a Getreidekaffee or "grain coffee") was not popular with the POWs, who longed for the real beverage.