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In political systems based on the principle of separation of powers, authority is distributed among several branches (executive, legislative, judicial)--an attempt to prevent the concentration of power in the hands of a single group of people. In such a system, the executive does not pass laws (the role of the legislature) or interpret them (the role of the judiciary). Instead, the executive enforces the law as written by the legislature and interpreted by the judiciary. The executive can be the source of certain types of law, such as a decree or executive order. Executive bureaucracies are commonly the source of regulations.
In political systems that use fusion of powers, which typically includes parliamentary systems, only the executive is typically referred to as the government (with the legislature often referred to as "Parliament" or simply "the legislature") which typically is either a part of or requires the confidence of (requires the support/approval of) the legislature and is therefore fused to the legislative power instead of being independent. In systems where the legislature is sovereign, the powers of and the organization of the executive are completely dependent on what powers the legislature grants it and the actions of the executive may or may not be subject to judicial review, something which is also controlled by the legislature. The executive may also have legislative or judicial powers in systems that where the legislature is sovereign, which is often why the executive is instead referred to as the government since it often possesses non-executive powers.
In parliamentary systems, the executive is responsible to the elected legislature, i.e. must maintain the confidence of the legislature (or one part of it, if bicameral). In certain circumstances (varying by state), the legislature can express its lack of confidence in the executive, which causes either a change in governing party or group of parties or a general election. Parliamentary systems have a head of government (who leads the executive, often called ministers) normally distinct from the head of state (who continues through governmental and electoral changes). In the Westminster type of parliamentary system, the principle of separation of powers is not as entrenched as in some others. Members of the executive (ministers), are also members of the legislature, and hence play an important part in both the writing and enforcing of law.
In this context, the executive consists of a leader or leader of an office or multiple offices. Specifically, the top leadership roles of the executive branch may include: