An emperor (through Old French empereor from Latin: 'imperator') is a 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), mother (empress dowager), or a woman who rules in her own right (empress regnant). Emperors are generally recognized to be of a higher honour and rank than kings. In Europe the title of Emperor was, since the Middle Ages, considered equal or almost equal in dignity to that of Pope, due to the latter's position as visible head of the Church and spiritual leader of Western Europe. The Emperor of Japan is the only currently reigning monarch whose title is translated into English as "Emperor".

Both kings and emperors are monarchs, but emperor and empress are considered the higher monarchical titles. In so far as there is a strict definition of emperor, it is that an emperor has no relations implying the superiority of any other ruler, and typically rules over more than one nation. Thus a king might be obliged to pay tribute to another ruler, or be restrained in his actions in some unequal fashion, but an emperor should in theory be completely free of such restraints. 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. For purposes of protocol, emperors were once given precedence over kings in international diplomatic relations; currently, however, precedence amongst heads of state – whether they be Kings, Queens, Emperors, or Presidents – is determined by the duration of time that each one has been continuously in office.