One of the most influential doctrines in history is that all humans are divided into groups called nations. It is an ethical and philosophical doctrine in itself, and is the starting point for the ideology of nationalism. The nationals are the members of the "nation" and are distinguished by a common identity, and almost always by a common origin, in the sense of ancestry, parentage or descent. The national identity refers both to the distinguishing features of the group, and to the individual’s sense of belonging to it. A very wide range of criteria is used, with very different applications. Small differences in pronunciation may be enough to categorize someone as a member of another nation. On the other hand, two people may be separated by difference in personalities, belief systems, geographical locations, time and even spoken language, yet regard themselves and be seen by others, as members of the same nation. Nationals are considered to share certain traits and norms of behavior, certain duties toward other members, and certain responsibilities for the actions of the members of the same nation.

Nations extend across generations, and include the dead as full members. More vaguely, they are assumed to include future generations. No-one fixes a timespan, but a nation is typically several centuries old. Past events are evaluated in this context, for instance by referring to "our soldiers" in conflicts which took place hundreds of years ago.

The term nation is often used synonymously with ethnic group (sometimes "ethnos"), but although ethnicity is now one of the most important aspects of cultural or social identity for the members of most nations, people with the same ethnic origin may live in different nation-states and be treated as members of separate nations for that reason. National identity is often disputed, down to the level of the individual.