An emperor is the ruler of an empire, which is a group of countries or territories united under one supreme authority. Emperors hold the highest possible rank among monarchs: they stand above kings and queens in power and prestige. While a king rules a kingdom, an emperor rules multiple kingdoms or vast territories, often spanning different peoples and cultures.

Throughout history, some of the world's most powerful rulers have been emperors. The Roman emperors controlled territories stretching from Britain to Egypt. The Chinese emperors ruled for thousands of years, with dynasties rising and falling across centuries. Napoleon crowned himself Emperor of France, and his empire briefly dominated much of Europe. Japan still has an emperor today, though the position is now ceremonial rather than governing.

Roman generals once earned the title imperator through military victories before it became the permanent title for Rome's supreme ruler. An emperor's wife is called an empress, though a woman can also rule as empress in her own right, like Russia's Catherine the Great.

The difference between emperor and king involves both scale and authority. Emperors traditionally claimed authority that transcended ordinary kingship: they might be seen as representatives of heaven, protectors of civilization, or unifiers of many peoples. When Japan's emperor was called the “Son of Heaven,” or when Roman emperors were treated as gods, these titles reflected the extraordinary status emperors held in their societies.