An acting president is an individual who fulfills the functions of the presidential office while no legal president is able to do so. The term "acting president" can apply either in the public or private sector. Acting presidents are generally sanctioned figures, not usurpers — their primary role is usually to maintain order during periods of uncertainty. Their tenure can range from just a few hours to months or even years, but is generally believed that their primary responsibility is to maintain normalcy during a transition or after a change.
In the public sector, the role of acting president is generally filled by an individual whose position is established by law. When an elected president is unable to fulfill his or her duties for any length of time, an acting president may be appointed. Acting presidents usually have the full rights and privileges of presidents. They typically must be able to make decisions for their countries in times of crisis.
In the US government, the role of acting president would usually be filled by the sitting vice president. According to the 25th Amendment, which established presidential succession, if a president of the United States will be unable to fulfill the duties of the office, he or she must notify Congress in writing so that the vice president may be named acting president. The law allows for the president to rescind that designation whenever he or she is ready to assume the duties of office again. The Amendment also permits Congress, by majority vote, to declare the President unfit to carry out his duties and permit the vice president to step in.
Many other countries have similar laws, and acting presidents have taken responsibility for governmental affairs in nations around the world when necessary. An acting president may be named merely to govern in the president's place during a routine surgical procedure, as the US Vice President Dick Cheney was. Sometimes, though, acting presidents come to power when a sitting president is killed, as was the case after the death of Polish president Lech Kaczynski, or pushed out of power, as with Tunisian president Zine El Abidine Ben Ali.
In the private sector, an acting president may be named to lead a company after a president resigns or is fired. Usually, the role is filled by someone who has a long history with the company and knows its operations well. She or he may be a contender for the formal position of president of the company, or may only be stepping in to ease the transition while the board of directors finds a suitable candidate.