The 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 or High King (harking back to the client kings of the Roman Empire). Although uncommon, king might refer to a client-king, where one lesser king swears fealty to another king. A good example would be where King Cadwy swore fealty to Uther, although Uther was no Emperor.
In the context of prehistory, antiquity and tribal peoples, the title may refer to tribal kingship. A good example for this is the kingship of Tisbury, who in ancient days is said to have been a kingship in itself in some matter, but in more modern context would hardly be considered even a barony.