An acting president temporarily wields presidential powers without holding the office, upholding the United States' leadership amidst transitions.
As of my last knowledge, there have been THREE individuals who have served as acting president of the United States:
On July 13, 1985, George Bush became acting president during Ronald Reagan's surgery, marking the first VP to temporarily assume presidential duties for about eight hours.
Cheney briefly acted as president on June 29, 2002, 7:09-9:24 a.m., invoking the 25th Amendment during George W. Bush's colonoscopy.
Harris, on Nov 19, 2021, briefly acted as president, a historic first for a woman, during Biden's colonoscopy, invoking the Twenty-fifth Amendment.