Found a definition which might be serviceable at the
diversity & social justice glossary:
White supremacy is a historically based, institutionally perpetuated system of exploitation and oppression of continents, nations and peoples of color by White people and nations of the European continent, for the purpose of maintaining and defending a system of wealth, power and privilege.
The only problem I can see with this approach is that it's a bit fuzzy at the edges of Europe, e.g. does the Ptolemaic Kingdom
WP count as such a system?