A king (monarch); can also refer to a queen in contexts where the term covers the sovereign.
国王が即位の演説を行った。
The king gave an accession speech.
昔の王国では、国王の権力が非常に強かった。
In old kingdoms, the king's power was very strong.
物語の中では若い国王が国を救う役割を果たす。
In stories, a young king often plays the role of saving the country.
In legal or formal contexts, 'the Crown' — referring to the sovereign as the focus of authority (e.g., in the UK).
その事件は国王に対する反逆と見なされた。
The incident was regarded as treason against the Crown.
イギリスでは、いくつかの公的手続きで国王（または女王）が形式的権限を持つ。
In the UK, the king (or queen) holds formal authority in certain official procedures.