"The Outside" is a catch-all term for that which exists beyond the limits of the known universe. This is not to be confused with parallel realities, alternate timelines, or even divergent universes--this is a realm where things completely alien to known reality exist.
The Outside is inspired by the same "beyond" that is frequently referenced in the Lovecraft Mythos
, though it is not identical.
The Dreamlands
The Dreamlands are an area deep in the Otherworld, which Outsiders are occasionally able to sneak through. However, this is at best a "slow leak," and they must be specifically ferried through via corrupted mortal Dreamers.