The US federal government is divided into three parts or branches: the Legislative branch (represented by the Senate and House of Representatives), the Judicial branch (represented by Supreme Court and lower Courts), and the Executive branch, (represented by the President and about 5,000,000 workers).
Regarding the Executive branch, all its power is vested in the President of the United State, whose duties and responsibilities include; the execution and enforcement of the laws created by Congress, appointing federal judges and ambassadors, as well as the heads the Cabinet and federal agencies, such as the Federal Reserve Board or the Securities and Exchange Commission and signing legislation into law or to veto bills enacted by Congress.
In addition, it is also a responsibility of the president, as head of the executive branch of government, conducting diplomacy with other nations, as well as negotiates and sign treaties with them.
Considering all the President’s duties, briefly described above, it could be said that the role that best describes him is <em>head of state and head of government of the United States of America.</em>