An excuse for the cause of bi-racial disagreements.

When one race thinks they're better than another race.

I would define it as discriminating against, or thinking less of, a person based on their race. I'm not the best at writing definitions though, that's just what I thought of off the top of my head.

Google definition - "the belief that all members of each race possess characteristics, abilities, or qualities specific to that race, especially so as to distinguish it as inferior or superior to another race or races."

Jep wrote When one race thinks they're better than another race.

That's only supremacy.
To truly define racism, one would need to say "prejudice, discrimination, or antagonism directed against someone of a different race based on the belief that one's own race is superior."