"The nonpartisan Counter Extremism Project defines “The Great Replacement Theory” here:

The Great Replacement Theory is an ethno-nationalist theory warning that an indigenous European—e.g., white—population is being replaced by non-European immigrants. The Great Replacement concept was popularized by French writer Renaud Camus in his 2012 book, Le Grand Replacement (“The Great Replacement”). Camus postulated that black and brown immigrants were reverse-colonizing native “white” Europeans."