Head of government
The head of government is the leader of the executive branch of a nation's government. This is not always the same person as the head of state. Common titles for heads of government include Premier or Prime Minister.
In some countries such as the United States, the president is both the head of state and the head of government. In the Republic of Ireland the president is the head of state but the Taoiseach (Prime Minister) is the head of government.
In most modern monarchies the king (or queen) is the head of state but not the head of government, as is the case of Japan and the UK, for example. In this scenario, the monarch will perform ceremonial duties while the head of government takes care of the executive branch.