Umvolkung (an artificial German term echoing Umpolung "polarity inversion", as it were translating to "ethnicity inversion") is a term in Nazi ideology used to describe a process of assimilation of members of the German people (the Volk) so that they would forget about their language and their origin.
The term is also utilized to describe the "re-Germanisation" of the German people, after new Lebensraum was conquered and the German people who already resided there would become more German again. Of course, Umvolkung in the first sense was seen as a negative process during the Third Reich, while the second process was seen as desirable.
Origin and Background 
The term was invented by Albert Brackmann, a prestigious leader of the Ostforschung, which was a research organization which investigated the character and the attitudes of people (the so-called "Verhalten") living in parts east of the German Reich, e.g. in Poland, Ukraine, Slovakia and Romania.
There was a plan to conquer almost whole Eastern Europe and process the "Umvolkung", so all the former German people, who slowly assimilated and mixed with the other ethnies would become more German again.
Today's use 
Nowadays, it is utilized by far-right German organizations and by people who identify with the Nazi ideology and its aspects, such as certain fraternities, parties, and organizations as the NPD or the Jungen Nationalen. The term became a catchphrase and is often utilized to describe German fears of Überfremdung by immigrants or their descendants whose numbers have been constantly increasing since the foundation of the German Federal Republic.