As /u/callalilly77 noted most people actually associate the word "nation" with the concept of a "nation-state".

A nation-state is a distinctly political entity - in that it consists of a government that rules over a defined geographical area that is confirmed and respected by other nation-states by international treaty. This model is traditionally attributed as becoming the standard in Europe after the Peace of Westphalia in 1648; which spread / was imposed worldwide due to European Imperialism.

It must also be noted that nation-states may have numerous distinct societies/cultures within its borders; which is why it's dangerous to assume that a nation-state automatically has a shared identity through culture, language, and history. This is particularly true in many former colonies whose borders were not defined by the people actually living in them.