A term used by Michael Moorcock (also, earlier and independently, by John Cowper Powys) to describe a universe consisting of innumerable Alternate Worlds, all intersecting, laterally and (Palimpsest-fashion) vertically. Some of these parallel worlds operate according to sf premises; some – like the worlds in which various Avatars of Moorcock's Eternal Champion series play out their linked destinies – operate in fantasy terms. Worlds governed by incompatible premises are not, however, barred from one another, and in this sense the overall concept belongs more properly to fantasy than to sf; Moorcock himself treats his extremely large and varied oeuvre as though all its venues occupy niches in the one multiverse. Novels in which Temporal Adventuress figures appear – Moorcock and others have written them – normally allow these figures free access to various Realities within the multiverse. The "polycosmos" featured in John Grant's work is analogous, but is depicted as having physical Reality, while one character is capable of travelling outside its constituent universes to view the polycosmos "from above".
Moorcock is not much interested in Portals or other fantasy devices of the sort. In his works, shifts from one world to another, whether or not voluntary, are normally signalled by shifts in Perception. To perceive a new world in the multiverse is to inhabit it.
According to the OED, the word itself was coined by William James (1842-1910) in 1895, and was also used by Sir Oliver Lodge (1851-1940) in 1904 and G K Chesterton in 1920. In this light, both Powys and Moorcock were johnny-come-latelies. [JC]