The president of the Church of Jesus Christ of Latter-day Saints is the only person in the movement held to be authorised to receive revelation binding on the whole body. He is sustained by the church twice a year as prophet, seer and revelator, presides over the First Presidency with two counsellors of his own choosing, and holds every priesthood key that anyone else in the church exercises, delegating them rather than sharing them.
The office joins three things that most religions keep apart. It is a teaching authority, and what the president says in general conference is treated as the current mind of God on a subject. It is the chief executive of an institution with a global membership, tens of thousands of congregations, a university system, a welfare and humanitarian operation and a very large investment portfolio. And it is a claim to living revelation, which means the office can do what no interpretive authority can: change what the church does. Both of the twentieth century's largest changes to Latter-day Saint practice, the end of plural marriage and the end of the priesthood restriction, were announced by a president and then canonised.
The office is held for life and there is no retirement. Presidents have often been very old, and in the twentieth century some were seriously incapacitated for extended periods, during which the counsellors in the First Presidency carried the administration. The church has never established a mechanism for a president to step down, and has not been asked to.