Merger is a term used to describe the changes exports and continuing series have on already imported characters.
Full merger Edit
A full merger is actually a sub-import, which means that alternate versions of characters are imported from their export universes. A true full merger has only happened with Phoenix Rebellion, an Avatar-export, and its follow-up Korra-export, which imported Aditi, Zula and Katta.
Later imports from the initial sources are also considered mergers, as seen with canon Korra in 2014 and Warriors in 2012.
In a sub-merger, no physical changes take place. Instead, characters already imported receive their canon versions' updated memories, or the full memories of their port version. Both have been observed to happen on Avatar and Korra characters after their ports caused full mergers, and also in MCU as their respective canons unfolded. Sly Cooper was also added via a sub-merger after Amy and Will's escapades in their world.
In 2028 Amy experienced the largest known sub-merger, which included two or three different Amy's memories, as well as multiple ports that only Mirror Amy went through, of which the most important ones are Forged Family and Blossoming Marble.