Interlopers are people from outside the realm, who somehow arrived in Oz. The name was first used by Sullivan and referred to Karen Chapman and her daughter Dorothy.
Interlopers are people who do not belong in Oz and come from different reality (or realities).
The Place In-between
Dorothy travelled to a strange place in-between two worlds when she entered a tornado at East's castle. It was a brook in a forest, filled with remnants from other interlopers - things they took with them when they arrived in Oz. Dorothy found her mother's lab coat floating in the brook.