Once upon a time, the term King is the formal title for the person whom is the head/leader of a nation or an Empire. After becoming a Democracy, the US decided to call their leader/head of state a President, instead of a King or an Emperor. After the establishment of constitutional monarchy structure, the head of state and leader of state can also be two different people in some nation. As a result, today, all democratic elected head of state/leader is call a President Democratic elected head of state whom are not leader of state is call President and democratic elected leader of state whom are not also head of state, is call a Prime Minister. Today, the term King or Emperor is usually a social term use to refer to the person whom is the leader and/or head of a nation. For example, in Australia, people sometime refer to the Prime Minister as the King, despite its constitutional head of state is the Queen of England. In Taiwan, the President is frequently been been refer to as an Emperor. Considering the US is call "United States" I am surprise nobody call the US President as King of Kings.