What is the best definition of the term nationalism?

I would say the efforts

Na·tion·al·ism

ˈnaSH (ə) nəˌlizəm/

noun

patriotic feeling, principles, or efforts.

synonyms:patriotism, patriotic sentiment, flag-waving, xenophobia, chauvinism, jingoism

"their extreme nationalism was frightening"

an extreme form of this, especially marked by a feeling of superiority over other countries.

plural noun: nationalisms

advocacy of political independence for a particular country.