king: the supreme ruler of a people or realm, the one in whom final authority rests
In the New Testament this is the ordinary word for a king or sovereign, from the local client-kings of Judea to the Roman emperor, and it keeps that plain political meaning wherever it names an earthly ruler. The single idea running through every use is supreme authority: the king is the one at the top, who commands, judges, wages war, and is owed allegiance.
What changes is not the meaning of the word but who is being called king. The Gospels press the political title onto Jesus, above all in the trial and crucifixion, where "King of the Jews" is at once a charge against him and, for the Gospel writers, the truth about him. Revelation and the letters then lift the same word to its highest reach, naming Jesus "King of kings" and naming God himself the King of the ages and of the nations.
So the senses below are not different definitions of the word. They are the same idea of kingship applied to three kinds of ruler: human kings, Jesus as king, and God as king. A reader uses the one English word "king" for all three; the difference is the one wearing the crown.