Monarchs, as such, bear a variety of titles — king or queen, prince or princess (e.g., Sovereign Prince of Monaco), emperor or empress (e.g., Emperor of Ethiopia, Emperor of Japan, Emperor of India), archduke, duke or grand duke (e.g., Grand Duke of Luxembourg), or sultan (e.g., Sultan of Oman). Prince is sometimes used as a generic term to refer to any monarch regardless of title, especially in older texts.
A king can also be a queen's husband and a queen can be a king's wife. If both people in a couple reign, neither person is generally considered to be a consort.