nomerci when one starts getting a sense of superiority or is handed down some sort of advantage just because of the colour of their skin or conversely if someone is discriminated against or made to feel inferior due to the colour of their skin.
That is just my simple definition, I just checked UN's definition and it is more complex and brings in everything into it. Race, ethnicity and geography and history and Biology and what not. Too complex to make sense of. Everyone is a racist as per that.