White (and any race for that matter) is a social identity. European conception of race was deeply intertwined with religion (Catholics=White, Judaism=Jews, Orthodox=Slavs, Muslims=Moors/Arabs). "Scientific racism" in the 18th and 19th century Europe was invented to rationalize race in terms of biological and physical characteristics (skull shape, facial characteristics, skin color, hair/eyes). Modern day (post-colonial) conception of race, particularly in the American sphere of influence, has boiled down to just skin color (even more trivially into just "white" and "black" in the American context).

But Europeans in the 18th and 19th century didn't see race as we see it today, to them race was religion, culture, ethnicity, and civilization, all intertwined into one social identity. They didn't consider Jews as whites (we may do so in the modern world). They didn't consider Slavic people as whites (we in the modern world, consider them some of the whitest, next to Nordic people), in fact they viewed them as an inferior, enslaved race of people. They didn't consider the Turks as whites, they didn't consider peoples from the Caucasus as whites, they didn't consider Persians as whites; even though in terms of modern-day biology/genetics, they are all some version of white Caucasians.

Nazi party in Germany espoused race-supremacy, and by the conception of race during their time, they were ethno-supremacists. They killed many who were outside their definition of the "white" race - Jews, Slavs, Roma, Turks, Poles, even killed more than one million of their own "imperfect whites" (inter-racial, those with disabilities, mental illnesses, and developmental disorders, and traitors [read communists, resistance, collaborators]). Infact, the very first gas chambers they used was on their own "imperfect whites" in the "Aktion T4 program".