I did not say it's limited to skin color--that's just the most obvious example. I agree that it's based on certain clusters of biological traits, and the discriminator's perception that those biological traits are accompanied by flaws in intellect, character, or something else. However, "racism" cannot exist without the cluster of biological traits we lazily refer to as "race."
If two 6'2" blond guys with blue eyes and freckles were before you, but one spoke like he's a Valley dude from north of LA, and the other spoke French and lived in Burgundy, you cannot be accused of racism for discriminating against the moron from Encino--even if your perception of his intellect is incorrect. Why? They're the same "race."
I'm done. Sometimes it seems people try not to understand.
Edit: Racism is one form of discrimination, but not all discrimination is racism. Gender, age, religion, nationality, sexual preference, whatever are all forms of discrimination. But we don't say someone is "racist against age," or against women, etc., because it's not based on "race."