The genre critic John Clute borrowed this term from the fourteenth-century Chinese novel Water Margin, or Outlaws of the Marsh, whose title evokes the wild, liminal edgelands surrounding the settled central region of the ruling power. Clute applies the term to the way Fantasy worlds often fade into undefined, unmapped or only partially imagined spaces at their edges. On Secondary World maps, such Water Margins might be represented by  illimitable ocean, desert, forest, mountain range or any similarly nebulous geographical feature marking
the edge of the described world.

Image: Ursula K. Le Guin’s ‘A Wizard of Earthsea‘ ©Ursula K. Le Guin/Ruth Robbins/Parnassus Press (1968). CC BY-NC-ND. Image courtesy of Ex Carta