King, Monarch, Sovereign
noun, の adjective
A country king is still a king, just of a bigger area.
こくおう
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
今のイギリスの国王はチャールズ三世です。
The current British king is Charles III.
日本には国王がいません。
There is no king in Japan.
国王の前でパフォーマンスをしたことがあります。
I have performed in front of the king.