This was the title in the Zhou dynasty for the single emperor who ruled the world, but when China entered the warring states period (China), the lords who were supposed to be the vassals of the king fought and called themselves 'king,' which led to an upsurge in the number of kings scattered throughout the country.
周代には天下を統治する唯一の天子として王の称号があったが、戦国時代_(中国)に入ると王の臣下であるはずの諸侯が争って「王」を自称したため、王が乱立した。