I'd say that when person A narrows the culture, religion, and physical characteristics of person B down to the basest caricature, and then rejects person B based on that caricature, that's as good a definition of racism as one might need.

It all comes down to denigrating someone for what they are, not for what they do. Unfortunately, our definition of 'race' has long been inexact so our definition of 'racism' is nebulous as a consequence. Am I "jewish?" According to the Law of Return I'm jewish, regardless of the fact that my grandmother disavowed her faith. Her husband's family were DAR. I have a great uncle that traced the other side of the family clear back to Old 300 so by any reasonable definition, I'm the most White Anglo Saxon Protestant American I know. Yet by Hitler's standards me and Sammy Davis Jr would both be lampshade fodder.

Except we all know that isn't true. I'd be fuckin' fine because I'm as Aryan a specimen as any Ubermensch could hope for. And Sammy Davis Jr would have been fucked regardless of who he prayed to. Because it's not about definitions, is it. It's about hatred. It's about short-circuiting your decision-making process when it comes to others because your life is easier to get through if you know who your enemies are.

You mentioned Potter Stewart earlier. Something people forget about Potter Stewart is that he wasn't arguing what pornography was, he was arguing what pornography wasn't:

I shall not today attempt further to define the kinds of material I understand to be embraced within that shorthand description; and perhaps I could never succeed in intelligibly doing so. But I know it when I see it, and the motion picture involved in this case is not that.

People forget that 20 years later the Supreme Court actually did come up with a definition of pornography:

1. whether the average person, applying contemporary "community standards", would find that the work, taken as a whole, appeals to the prurient interest;

2. whether the work depicts or describes, in an offensive way, sexual conduct or excretory functions, as specifically defined by applicable state law (the syllabus of the case mentions only sexual conduct, but excretory functions are explicitly mentioned on page 25 of the majority opinion); and

3. whether the work, taken as a whole, lacks serious literary, artistic, political, or scientific value.

That, of course, led to predictable amounts of margin-sniping:

A companion case to Miller, Paris Adult Theatre I v. Slaton, provided states with greater leeway to shut down adult movie houses. Controversy arose over Miller's "community standards" analysis, with critics charging that Miller encouraged forum shopping to prosecute national producers of what some believe to be "obscenity" in locales where community standards differ substantially from the rest of the nation. For example, under the "community standards" prong of the Miller test, what might be considered "obscene" in Massachusetts might not be considered "obscene" in Utah, or the opposite might be true; in any event, prosecutors tend to bring charges in locales where they believe that they will prevail. Justice Brennan, author of the Roth opinion, argued in his dissent for Paris Adult Theatre that outright suppression of obscenity is too vague to enforce in line with the First and Fourteenth Amendments.

Which, of course, leads to people arguing about what "community" is and eventually having whole sections of laws struck down because fuckin' hell, it's a violation of first amendment rights.

And here we are. Progressing. With a finely refined test for what "obscenity" is and a growing consensus that we can't constitutionally do anything about it.

Aryan. Know what that used to mean?

Kinda weird that a bunch of racist Germans wanted nothing more in the world than to be Persian. Except of course they didn't, they'd just managed to erroneously think that whatever language they spoke obviously had roots older than anything else and since they were demonstrably superior that meant that they laid claim to it despite the fact that it meant they were expressing racial kinship with this guy:

Because really? Racists care fuckall about the precision of their racism except as it allows them to claim to not be racist. It's useful, though, to be dragged into arguments about the definition of racism, the boundaries of racism, the contraindicators of racism and the exceptions of racism if it in any way keeps us from making racists uncomfortable about their racism.

To which I say fuck that shit.