An emperor (through Old French empereor from Lain imperator) is a (male) monarch, usually the sovereign ruler of an empire or another type of imperial realm. Empress, the female equivalent, may indicate an emperor's wife (empress consort) or a woman who rules in her own right (empress regnant). Emperors are generally recognized to be of a higher honor and rank than kings.
Both kings and emperors are monarchs. Within the European context, emperor and empress are considered the higher monarchical titles. However, monarchs heading empires have not always used the title—the British sovereign did not assume the title until the incorporation of India into the British Empire, and even then used it only in a limited context. Emperors were once given precedence over kings in international diplomatic relations; currently, precedence is decided by the length a head of state is continuously in office.
Outside the European context, emperor is a translation given to holders of titles who are accorded the same precedence as European emperors in diplomatic terms. In reciprocity, these rulers may accredit equal titles in their native languages to their European peers. Through centuries of international convention, this has become the dominant rule to identifying an emperor in the modern era.
Historians have liberally used emperor and empire anachronistically and out of its Roman and European context to describe any large state and its ruler in the past and present. Empire became identified with vast territorial holdings rather than the title of its ruler by the mid-18th century.
Put simply, an emperor rules over not only his own country but others which have been conquered or assimilated and together with his country form an empire. Often these other countries are subordinate to the main country. For example, the Roman Emperor ruled over Italy as well as many other countries in Europe, Asia, and Africa which had been conquered by the Romans and assimilated into the Roman Empire. A king, in contrast, rules over his own country; e.g., the King of France rules over France only.