The main difference between Emperor and King is that the Emperor is a type of monarch and King is a class of male monarch
An emperor (through Old French empereor from Latin imperator) is a monarch, and 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 has been used since the Middle Ages, considered in those times 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 the Catholic part of Western Europe. The Emperor of Japan is the only currently reigning monarch whose title is translated into English as Emperor.
Both emperors and kings are monarchs, but emperor and empress are considered the higher monarchical titles. Inasmuch 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, therefore 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 in all contexts—the British sovereign did not assume the title Empress of the British Empire even during the incorporation of India, though she was declared Empress of India.
In Western Europe, the title of Emperor was used exclusively by the Holy Roman Emperor, whose imperial authority was derived from the concept of translatio imperii, i.e. they claimed succession to the authority of the Western Roman Emperors, thus linking themselves to Roman institutions and traditions as part of state ideology. Although initially ruling much of Central Europe and northern Italy, by the 19th century the Emperor exercised little power beyond the German-speaking states.
In India, the Muslim Mughal Emperors were the only South Asian rulers for whom the term was consistently used by Western contemporaries.
Although technically an elective title, by the late 16th century the imperial title had in practice come to be inherited by the Habsburg Archdukes of Austria and following the Thirty Years’ War their control over the states (outside the Habsburg Monarchy, i.e. Austria, Bohemia and various territories outside the empire) had become nearly non-existent. However, Napoleon Bonaparte was crowned Emperor of the French in 1804 and was shortly followed by Francis II, Holy Roman Emperor, who declared himself Emperor of Austria in the same year. The position of Holy Roman Emperor nonetheless continued until Francis II abdicated that position in 1806. In Eastern Europe, the monarchs of Russia also used translatio imperii to wield imperial authority as successors to the Eastern Roman Empire. Their status was officially recognised by the Holy Roman Emperor in 1514, although not officially used by the Russian monarchs until 1547. However, the Russian emperors are better known by their Russian-language title of Tsar even after Peter the Great adopted the title of Emperor of All Russia in 1721.
Historians have liberally used emperor and empire anachronistically and out of its Roman and European context to describe any large state from the past or the present. Such pre-Roman titles as Great King or King of Kings, used by the Kings of Persia and others, are often considered as the equivalent. Sometimes this reference has even extended to non-monarchically ruled states and their spheres of influence such as the Athenian Empire of the late 5th century BC, the Angevin Empire of the Plantagenets and the Soviet and American “empires” of the Cold War era. However, such “empires” did not need to be headed by an “emperor”. Empire became identified instead with vast territorial holdings rather than the title of its ruler by the mid-18th century.
For purposes of protocol, emperors were once given precedence over kings in international diplomatic relations, but currently precedence amongst heads of state who are sovereigns—whether they be kings, queens, emperors, empresses, princes, princesses and to a lesser degree presidents—is determined by the duration of time that each one has been continuously in office. Outside the European context, emperor was the translation given to holders of titles who were accorded the same precedence as European emperors in diplomatic terms. In reciprocity, these rulers might 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.
King is the title given to a male monarch in a variety of contexts. The female equivalent is queen regnant in (while the title of queen on its own usually refers to the consort of a king).
In the context of prehistory, antiquity and contemporary indigenous peoples, the title may refer to tribal kingship. Germanic kingship is cognate with Indo-European traditions of tribal rulership (c.f. Indic rājan, Gothic reiks, and Old Irish rí, etc.)
In the context of classical antiquity, king may translate Latin rex or either Greek archon or basileus.
In classical European feudalism, the title of king as the ruler of a kingdom is understood as the highest rank in the feudal order, potentially subject, at least nominally, only to an emperor (harking back to the client kings of the Roman Empire).
In a modern context, the title may refer to the ruler of one of a number of modern monarchies (either absolute or constitutional). The title of king is used alongside other titles for monarchs: in the West, prince, emperor, archduke, duke or grand duke, and in the Middle East, sultan or emir; etc.
King may also refer to a king consort, a title that is sometimes given to the husband of a ruling queen, however the title prince consort is sometimes granted instead.
The male monarch or ruler of an empire.
Specifically, the ruler of the Holy Roman Empire; the world-monarch.
“The Investiture Controversy was a conflict between the Emperor and the Pope.”
The fourth trump or major arcana card of the tarot deck.
A large, relatively valuable marble in children’s games.
Any fish of the family ver=180504.
Any various butterflies of the subfamily subfamily.
A male monarch; a man who heads a monarchy. If it’s an absolute monarchy, then he is the supreme ruler of his nation.
“Henry VIII was the king of England from 1509 to 1547.”
A powerful or majorly influential person.
Something that has a preeminent position.
“In times of financial panic, cash is king.”
A component of certain games.
The principal chess piece, that players seek to threaten with unavoidable capture to result in a victory by checkmate. It is often the tallest piece, with a symbolic crown with a cross at the top.
A playing card with the letter “K” and the image of a king on it, the thirteenth card in a given suit.
A king skin.
“Oi mate, have you got kings?”
A male dragonfly; a drake.
A king-sized bed.
To crown king, to make (a person) king.
To rule over as king.
To perform the duties of a king.
To assume or pretend preeminence (over); to lord it over.
To promote a piece of draughts/checkers that has traversed the board to the opposite side, that piece subsequently being permitted to move backwards as well as forwards.
To dress and perform as a drag king.
a sovereign ruler of an empire
“he is regarded as one of the greatest Roman emperors”
“he became emperor in 1930”
an orange and brown North American butterfly with a swift dodging flight, breeding chiefly on hackberries.