Nationalism is a regional attitude introduced in the Migration DLC. It represents how ethnocentric and jingoistic a region is. Highly nationalist regions denounce any form of immigration into their land and are supportive of policies which close it off to outsiders.
Lowering nationalist attitudes involves exposing a region's population to other people and ways of life. There are three policies which reduce nationalism; a Job-Sharing Initiative, Educational Enrolment, and Emergency Housing Relief.
If nationalism gets too high in a consumerist region, there's a chance that far-right political parties will take control of the region, drastically increasing the region's rejection of immigration.