A presidential system elects its head of state and head of government, one person holding both roles, separately from its legislature, and fixes that president's term so the legislature cannot remove them through an ordinary vote the way a parliamentary system can dismiss a prime minister. Executive and legislative power are meant to check each other precisely because neither depends on the other for its survival in office.
The United States Constitution of 1789 built the first modern presidential system on that separation of powers, deliberately rejecting the fused executive and legislature of Britain's parliamentary model the former colonies had just broken from. James Madison and the Constitution's other framers designed the presidency, Congress and the courts to check one another by structure rather than by convention, so that no single branch could accumulate the kind of unaccountable authority the colonies associated with the crown.
The design has since become one of the most widely copied constitutional templates outside its country of origin, adopted with local variations across Latin America, West Africa and elsewhere. A peaceful transfer of power between an outgoing and incoming president, resolved by a fixed election calendar rather than by a legislature's changing confidence, remains the system's central practical test.