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).

Historically the term "king" has been used for the rulers of very small peoples and territories. Such a king (Sumerian lugal, Semitic sharrum, Latin rex, Greek basileus, Sanskrit raja, Germanic kuningaz) could be a tribal leader or chief, or the tyrant of a city state. Often, the king will not only have a political function, but the same time a religious one, acting as high priest or divine king.

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.

In rare cases women have been crowned as kings instead of queens, such as Jadwiga of Poland.