A king is a head of state, who may or may not, depending on the style of government of a nation, exercise monarchal powers over a nation, usually called a kingdom or a realm. A king is the second highest sovereign title, only looking up to an emperor. The female equivalent of king is queen; although the term "queen" may refer to one ruling as a monarch in her own right, a queen regnant, or to the wife of a king, a queen consort. A queen who becomes the reigning monarch because the king has died, has become debilitated, or is a minor, is known as a queen regent. The husband of a queen regnant is sometimes styled the king consort but is more commonly styled the prince consort. A king or queen may wear a crown or carry other regalia (symbols of office).

Terms for kings can vary (Sumerian lugal, Semitic melech, Celtic rix, Latin rex, Greek basileus, Sanskrit raja, Germanic kuningaz) and in some cases can be a tribal leader or chief, or the tyrant of a city state. Tribal leaders continue to be referred to as king also into the modern period, e.g. Maquinna, king of perhaps 2000 Nootka people in the early 20th century.

Often, the king will not only have a political function, but also a religious one, acting as high priest or divine king.

In rare cases women have been crowned as king instead of queens, such as Jadwiga of Poland and Christina of Sweden.

A government that is completely under a kings or queen's rule is called an absolute monarchy (such countries include Saudi Arabia and United Arab Emirates). A government that has a king or queen with limited power is called a constitutional monarchy (such countries include Canada, the United Kingdom and Japan).