Nationalism is an ideology, a sentiment, a form of culture, or a social movement that focuses on the nation. According to Blank and Schmidt (2003) it is an idealisation of a nation, with a sense of uncritical acceptance and belief in superiority, a tendency to define one's group by descent, race or culture and regarding one's own group as homogenous and being derogatory about the other group. It is a type of collectivism emphasizing the collective of a specific nation.

While there is significant debate over the historical origins of nations, nearly all specialists accept that nationalism, at least as an ideology and social movement, is a modern phenomenon originating in Europe.

The term was coined by Johann Gottfried Herder (nationalismus) during the late 1770s. Precisely where and when it emerged is difficult to determine, but its development is closely related to that of the modern state and the push for popular sovereignty that came to a head with the French Revolution in the late 18th century. Since that time, nationalism has become one of the most significant political and social forces in history, perhaps most notably as a major influence or cause of World War I and especially World War II with the rise of fascism, a radical and authoritarian nationalist ideology.

As an ideology, nationalism holds that 'the people' in the doctrine of popular sovereignty is the nation, and that as a result only nation-states founded on the principle of national self-determination are legitimate. Since most states are multinational, or at least home to more than one group claiming national status, in many cases nationalist pursuit of self-determination has caused conflict between people and states including war (both foreign and domestic), secession; and in extreme cases, genocide.

Nationalism is a strong social phenomenon in the world as national flags, national anthems and national divisions are examples of 'banal' nationalism that is often mentally unconscious. Moreover, some scholars argue that nationalism as a sentiment or form of culture, sometimes described as 'nationality' to avoid the ideology's tarnished reputation, is the social foundation of modern society. Industrialization, democratization, and support for economic redistribution have all been at least partly attributed to the shared social context and solidarity that nationalism provides. Some scholars see its use as pejorative, standing in opposition to a more positive term, patriotism.

Even though nationalism ultimately is based on supporting one's own nation, nationalists of different states may perfectly well cooperate among each other as to support the ultimate worldwide belief that all groups of nationalities have the right to have their own states.

Nevertheless, nationalism remains a hotly contested subject on which there is little general consensus. The clearest example of opposition to nationalism is cosmopolitanism, with adherents as diverse as liberals, Marxists, anarchists, and some groups oppose nationalism based on religious beliefs. Even nationalism's defenders often disagree on its virtues, and it is common for nationalists of one persuasion to disparage the aspirations of others for both principled and strategic reasons.