For many critics and readers, there is little or no distinction between an otherworld and a Secondary World. The two terms are indeed often used interchangeably in this encyclopedia to designate autonomous worlds that are not bound to the mundane world (the lands of the dead from which Ghosts make their forays, and other territories common to Supernatural Fiction, are therefore excluded) and which are impossible in terms of our normal understanding of the sciences and of history (the worlds described in Planetary Romances are not, for instance, otherworlds or secondary worlds) and which are self-coherent in terms of Story (that is, they exist in a time and place and Reality in which stories can be told, believed and returned to – like the Middle-Earth created by J R R Tolkien).
There are, however, two distinctions which may be made; each should be clear in context. (a) The term "otherworld" may refer to any sort of autonomous impossible world, including Faerie and Wonderlands, while the secondary world is not normally thought of as being governed by the arbitrary rules that, for instance, operate the wonderlands of Lewis Carroll. (b) A secondary world may or may not have some sort of connection to the mundane world through, for instance, the Portals and Crosshatches which are so common in fantasy, but otherworlds certainly have some sort of connection with mundane reality. C S Lewis's Narnia is a secondary world; but its protagonists are constantly to-ing and fro-ing between one world and the other, and so it can also be described as an otherworld. [JC]