Here's why:
* It names a thing: Segregation refers to the act or practice of separating people into different groups based on race, ethnicity, gender, etc.
* It can be the subject or object of a sentence:
* Segregation was a major issue in the American South. (Subject)
* The government enacted laws to end segregation. (Object)
Let me know if you have any other words you'd like help identifying the part of speech for!