In the United States, the presidents are elected through something called the electoral college. The winning candidate has to win a majority of the electoral votes before they are elected president. Even though electors are typically tied to the popular vote, there has been some down in the past about whether electors are going to cast their vote for the candidate who won the state. Faithless electors have never turned an election, so the process remains. Another rule in place is that a president can only serve for two terms. If a president is elected, he or she serves for a time of four years. If they win another term, then they serve for a total of eight years, and they are not allowed to run again.
Even though there are presidents who have served for a total of eight years, not all presidents last that long. For example, the shortest serving president in the history of the United States, William Henry Harrison, only served for approximately one month before he passed away. Therefore, he is the shortest serving president in the history of the United States.
Right now, there is a constitutional amendment in place that limits the time someone can serve as president to eight years. Even though there is a firm rule in place right now, that was not always the case. All other presidents voluntarily decided to step aside because George Washington also voluntarily stepped aside after eight years. The first person to break that trend was Franklin Delano Roosevelt, who was elected to four terms as president. He led the United States through World War II, and the only reason why he stepped aside is that he passed away in office. Even though he is universally beloved because he not only won World War II but also pulled the country out of the Great Depression , people thought it was a bad precedent to have someone serve as long as he liked. Therefore, after he passed away, a constitutional amendment was passed that limits presidents to serving for eight years or two terms.
It is also possible for someone to serve for a single term as president and not be reelected. The last president to serve for a single term was Donald Trump, who was defeated by Joe Biden in the presidential election in 2020. Prior to him, the last president to only serve one term was George HW Bush, who served as President from 1988-1992. Prior to him, the previous single-term president was Jimmy Carter. He served as president from 1976 to 1980. In 1980, Ronald Reagan won the election in a landslide. In between George HW Bush and Donald Trump, every other president served a total of two terms. It remains to be seen if Joe Biden will run for a second term.