I went to anti-racism training for an organization with which I volunteer, and the people who ran the training said that white people generally define racism as mean people who do mean things where race is involved in some way, where people of color define racism as systems and structures that promote white supremacy, and then also people's actions which support it.
I think most white people define racism in whatever way most benefits white people, and that is in a way that absolves them of any part of it. Saying it's just mean people means it's someone else and they don't have to think about it. But when you recognize it as systems and structures that we're all a part of, then we have to look at ourselves and our own lives to see how those systems and structures work differently for us because of our race, and what part we might take in those systems and structures to maintain them as they are.
White people often see blame when they think of racism because they think it's about mean people who do mean things - so if you say they're a part of it they think you're saying they're mean people. Calling a white person "racist" is about the biggest insult there is, and again I think that's intentional so that we can't talk about it.
And then that leads to white people thinking people are saying they should feel guilty. If someone says you're a mean person, of course you should feel guilty. And we can say as many times as we want that we're talking about responsibility because we are part of these systems and structures, not guilt because we're bad people, but it seems to be ignored or not heard or something.