theantijock saidI'd always thought it was just no more than 2 consecutive terms.
But according to
Question: How Many Years Can a Person Serve as President of the United States?
Answer: A president is limited to serving for 10 years in office. He or she can only be elected to two full terms according to the 22nd amendment to the US Constitution. However, if an individual becomes president through the order of succession, then they are allowed to serve an additional two years.
Think JFK and LBJ.
LBJ served the remainder of JFKs term, and since it was less than two years LBJ could have served two full terms. Had the 22nd amendment not been ratified in 1951, Truman could have served the remaining three years and half years of FDRs last term, which he did, and run for reelection for as many terms as he could win. Because the 22nd was ratified while he was in office, Truman was eligible to run for reelection only once from 1948 to 1952