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.
あの国王は、とてもよくばりです。
That king is very greedy.
わたしは、国王のことをじょじょにりかいしてきました。
I'm starting to understand the king.
18歳という若さで国王になった。
I became the king as a youth of 18.