The Emperor's new clothes.
Essentially it's the claim that being White affords one access to universal, institutional or systemic advantages, used by anti-Whites (who use the code-word "anti-racist" to disguise their intentions) to justify overt racial discrimination against Whites, in the hopes of equalizing social and economic outcomes between races, whose disparities are better explained by hereditary differences in intelligence and temperament.
You cannot measure it, and it has no specific concrete definition to provide a basis for proof of its existence, it has not, and can never be proven to exist, but if you question it you're called "ignorant" (in reality you are a skeptic), and the fact that you questioned it in the first place is used as further proof that you have "White privilege".
As seen below the examples of so called "White privilege" are all either statistics presented without context, usually as part of an argumentum ad ignorantum, (disparities in callbacks/hiring is presumed to be racially discriminatory only because an alternative hypothesis is not considered. There is zero evidence for it.) or a meaningless generalisation like:
"-I am never asked to speak for all the people of my racial group."
Which is correct for 99.999% of all people everywhere, regardless of race.
"-I can if I wish arrange to be in the company of people of my race most of the time."
Which again is true for everyone, unless they have gone out of their way to move into a racially alien neighborhood.
Or even the following:
"-I can easily find academic courses and institutions which give attention only to people of my race."
Which is an out and out lie, as it is exactly the opposite of objective reality where affirmative action and quotas, and minority studies classes greatly disadvantage Whites in a way that is measurable, and where the causal relationship is clear to even the most casual observer.