King is the title given to a male monarch in a variety of contexts. The female equivalent is queen, which title is also given 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 , Gothic rājan , and Old Irish reiks , etc.). rí In the context of classical antiquity, king may translate in Latin as
and in Greek as rex or archon . basileus In classical European
feudalism, the title of king as the ruler of a kingdom is understood to be 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 Republic and 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, emperor, grand prince, prince, archduke, duke or grand duke, and in the Islamic world, malik, sultan, emir or hakim, etc.  The city-states of the Aztec Empire had a Tlatoani, which were kings of pre-Hispanic Mesoamerica. The Huey Tlatoani was the emperor of the Aztecs. 
Charlemagne or Charles the Great (748–814) was King of the Franks, King of the Lombards, and the first Holy Roman Emperor. Due to his military accomplishments and conquests, he has been called the "Father of Europe".
Heraldic crown of the King of the Romans (variant used in the early modern period)
The Iron Crown of the Lombards, a surviving example of an early medieval royal crown
12th-century depiction of Theodoric the Great, King of the Ostrogoths.
Louis XIV of France, the "Sun King" ( Roi-Soleil), who ruled at the height of French absolutism (painting by Hyacinthe Rigaud, 1701).
Aztec King Nezahualpiltzintli of Texcoco
king may also refer to a king consort, a title that is sometimes given to the husband of a ruling queen, but the title of prince consort is sometimes granted instead.