In a parliamentary or semi-presidential system of government, a reserve power is a power that may be exercised by the head of state without the approval of another branch of the government. Unlike in a presidential system of government, the head of state is generally constrained by the cabinet or the legislature in a parliamentary system, and most reserve powers are usable only in certain exceptional circumstances. In some countries, reserve powers go by another name; for instance, the reserve powers of the President of Ireland are called discretionary powers.