What custom. Eventually became part of constitution as the 22nd amendment

The custom which eventually became part of the Constitution as the 22nd amendment is that no president can be elected for a third term.

This means that an American politician can be chosen only twice to serve as president for the duration of 8 years in total, if chosen twice, or 4 years, if chosen once. This was the custom since the creation of America, but Franklin D Roosevelt broke this rule and served for 4 terms, after which the 22nd amendment was created.