Simple Definition:

Nationalism is the belief that a particular nation is superior to others and that its interests, culture, and values should be prioritized [favored] over those of other nations. It can also refer to a political ideology that emphasizes national sovereignty [independence] and self-determination [the right to make decisions for oneself]. Examples of nationalism include celebrating national holidays, displaying national symbols, and advocating for policies that prioritize the interests of one’s own country.

Very Simple Definition:

Nationalism is when people think their country is the best [better than others] and they want to do what’s best for their country. For example, they might want their country to be strong and independent [able to make their own choices]. People show nationalism by celebrating their country’s special days, like Independence Day, and by having symbols that represent their country, like a flag.