An interloper is an individual from Earth who somehow arrived in Oz, generally unwanted by the locals.
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.
- Even though Dorothy is considered an interloper, she technically is not one, as she was born in Oz.