Clearly any Secondary World or Otherworld or Wonderland can be thought of as an alternate world. By definition, none of these regions are of this world, nor are they arguable extrapolations of the history of this world. They are other. Upon entering them, readers experience a clear sense of Alterity. The realities – the ground rules – that govern them are alternate to those of the real world. At the same time, to treat any fantasy set wholly or partially in such a world is effectively to treat most fantasy and the AW story as synonymous. In this encyclopedia the term is used in a more restricted sense, one that works in tandem with Brian Stableford's definition of it in an sf context: "An alternate world" – as he phrases it in SFE – "is an account of Earth as it might have become in consequence of some hypothetical alteration in history." In fantasy, an alternate world is an account of our world as it might otherwise have been.
The difference is simple, but crucial. If a story presents the alteration of some specific event as a premise from which to argue a new version of history – favourite "branch points" include the victory of the Spanish Armada in 1588, the victory of the South in the American Civil War, and Hitler Wins scenarios – then that story is likely to be sf. If, however, a story presents a different version of the history of Earth without arguing the difference – favourite differences include the significant, history-changing presence of Magic, or of actively participating Gods, or of Atlantis or other Lost Lands, or of Crosshatches with Otherworlds – then that story is likely to be fantasy.
There is one obvious exception to this pattern of distinctions: the story in which a specific but not arguable event occurs to change the course of history. An example is the survival of Count Dracula, and his subsequent marriage to Queen Victoria, in Kim Newman's Anno Dracula (1992). As Vampires do not exist, the history-changing triumph of the Count does not constitute an alteration in the outcome of a specific historical event. It is, rather, an intervention into world history, and because it violates history a tale like Newman's is more likely to be a Supernatural Fiction than a fantasy proper.
Fantasies of History – in which, typically, Secret Masters manage the world – might seem to constitute a further exception, but in fact do not. The fantasy of history does not present an AW: it argues that it is this world which is different from what we think, and that the difference has been concealed from us. To treat such stories as AW tales is to deprive them of their paranoid bite.
There is a subtle distinction between the AW tale and the story of Alternate Realities. The AW tale depicts a world that is related to our own, but different for one reason or another. The alternate-reality tale conceives that there are other Realities which may or may not be accessible from or interact with our own (although they usually are and/or do, because that is generally the crux of the tale) but which have no historical dependence on it and may be totally dissimilar from it. Such realities may occur because "total reality" is a far more complex construct than we are aware, as in Thomas Palmer's Dream Science (1990), or they may be realities of the mind rather than having physical existence (although who is to decide their physical status?), as in Gene Wolfe's There Are Doors (1988). Either way, such stories are likely to involve Perception, whereas in AW tales there is no mysterious other layer of existence to be unravelled, and characters' perceptions of the hovering presence of an AW, if they are perceiving both this world and the AW simultaneously, are likely to be definable as Trompe-L'oeil.
One further type of AW story (although it can be read also as an Alternate-Reality story) can be exemplified by Ken Grimwood's Replay (1986), in which the central characters re-experience the same chunk of history several times over, changing it through their actions both in detail or in the large. The rationale that emerges is that Time itself has "knots", and that they are the only ones who can perceive this particular "knot cluster". The "knotting" of time has thus created a series of AWs, but these are, if properly perceived, sequential rather than parallel, as in the more usual AW story. (The movie Groundhog Day  depicts a similar recycling through time, but it is not clear if this is due to time "repeating" or to the protagonist in effect timeslipping.)
AW fantasy texts include: Joan Aiken's Willoughby Chase sequence; Brian W Aldiss's The Malacia Tapestry (1976); Poul Anderson's Operation Chaos (coll of linked stories 1971) and A Midsummer Tempest (1974); Avram Davidson's Vergil sequence (1969-1987), Peregrine sequence (1971-1981) and The Enquiries of Dr Eszterhazy (coll 1975); Peter Dickinson's The Blue Hawk (1976) and King and Joker (1976), with the latter's sequel Skeleton-in-Waiting (1989); Robert A Heinlein's "Magic, Inc" (1942 Unknown as "The Devil Makes the Law"); Robert E Howard's Conan books (in book form from 1950); Diana Wynne Jones's Chrestomanci sequence (1977-1988); Katherine Kurtz's Deryni books (from 1970); Ursula K Le Guin's Orsinian Tales (coll 1976) and Malafrena (1979); Last Letters from Hav (1985) by Jan Morris (1926-2020); A Time to Choose (1973) by Richard Parker (1914-1990), whose protagonists inhabit two versions of the world; Milorad Pavič's Hazarski recnik (1988; trans as Dictionary of the Khazars 1988); and Roger Zelazny's Amber sequence (from 1970). [JC]