General synonyms:
* Bias
* Prejudice
* Bigotry
* Prejudgement
* Unfairness
* Inequality
* Exclusion
* Segregation
* Oppression
More specific synonyms:
* Racism (discrimination based on race)
* Sexism (discrimination based on sex)
* Ageism (discrimination based on age)
* Homophobia (discrimination based on sexual orientation)
* Xenophobia (discrimination based on nationality or ethnicity)
Formal synonyms:
* Disparity
* Differentiation (when used negatively)
* Categorization (when used negatively)
The best synonym to use will depend on the context and the specific type of discrimination you are referring to.