Same character can be played with on multiple worlds/servers without losing the things they take from the others. Likely goes unexplained in-mythos.
Alternate name: Cross-server characters
The first video game about Cross-world characters was released on December 1996.
This makes only sense when you actually get to keep your inventory, character level, attributes, skills, etc. from the other worlds. Otherwise it's practically no different from having separate characters on them.
Implies the characters are stored locally on player's computer or in shared online storage all servers have access to.