CB, it was in my post, informally ... "interacting with people differently based entirely on the colour of their skin... I mean, how else do you define racism?"
If you want a formal, short definition, try this one from Oxford languages, as provided by Google when you ask it to define racism: prejudice, discrimination, or antagonism by an individual, community, or institution against a person or people on the basis of their membership of a particular racial or ethnic group, typically one that is a minority or marginalized.
For more detail, try the Wiki entry:
https://en.m.wikipedia.org/wiki/Racism