In answer to a question posed by a friend:
According to the Oxford English Dictionary, the word “multiverse” was first used by the American psychologist William James (the brother of novelist Henry James) in 1895. But he was interested in mysticism and religious experiences, not the nature of the physical Universe. Similarly, although the word appears in the writings of G. K. Chesterton, John Cowper Powys, and Michael Moorcock, none of this has any relevance to its use in a scientific context. From our point of view, the first intriguing scientific use of the word followed from an argument put forward by Alfred Russel Wallace, the man who came up with the idea of evolution by natural selection independently of Charles Darwin, that “our earth is the only inhabited planet, not only in the Solar System but in the whole stellar universe.” Wallace wrote those words in his book Man’s Place in the Universe, published late in 1903, which developed ideas that he had previously aired in two newspaper articles. Unlike Darwin, Wallace was of a religious persuasion, and this may have coloured his judgement when discussing “the supposed Plurality of Worlds.” But as we shall see, there is something very modern about his approach to the investigation of the puzzle of our existence. “For many years,” he wrote:
I had paid special attention to the problem of the measurement of geological time, and also that of the mild climates and generally uniform conditions that had prevailed throughout all geological epochs, and on considering the number of concurrent causes and the delicate balance of conditions required to maintain such uniformity, I became still more convinced that the evidence was exceedingly strong against the probability or possibility of any other planet being inhabited.
This was the first formal, scientific appreciation of the string of coincidences necessary for our existence; in that sense, Alfred Russel Wallace should be regarded as the father of what is now called “anthropic cosmology.”
Wallace’s book stirred up a flurry of controversy, and among the people who disagreed publicly with his conclusions were H. G. Wells, William Ramsay (co-discoverer of the inert gas argon), and Oliver Lodge, a physicist who made pioneering contributions to the development of radio. It was Lodge who used the term “multiverse,” but referring to a multitude of planets, not a multitude of universes.
In scientific circles, the word was forgotten for more than half a century, then invented yet again by a Scottish amateur astronomer, Andy Nimmo. In December 1960, Nimmo was the Vice Chairman of the Scottish branch of the British Interplanetary Society, and was preparing a talk for the branch about a relatively new version of quantum theory, which had been developed by the American Hugh Everett. This has become known as the “many worlds interpretation” of quantum physics, with “world” now being used as a synonym for “universe.” But Nimmo objected to the idea of many universes on etymological grounds. The literal meaning of the word universe is “all that there is,” so, he reasoned, you can’t have more than one of them. For the purposes of his talk, delivered in Edinburgh in February 1961, he invented the word “multiverse” – by which he meant one of the many worlds. In his own words, he intended it to mean “an apparent Universe, a multiplicity of which go to make up the whole . . . you may live in a Universe full of multiverses, but you may not etymologically live in a Multiverse of ‘universes’.”
Alas for etymology, the term was picked up and used from time to time in exactly the opposite way to the one Nimmo had intended. The modern usage of the word received a big boost in 1997, when David Deutsch published his book The Fabric of Reality, in which he said that the word Multiverse “has been coined to denote physical reality as a whole.” He says that “I didn’t actually invent the word. My recollection is that I simply picked up a term that was already in common use, informally, among Everett proponents.” In my books, the word “Multiverse” is used in the way Deutsch defines it, which is now the way it is used by all scientists interested in the idea of other worlds. The Multiverse is everything that there is; a universe is a portion of the multiverse accessible to a particular set of observers. “The” Universe is the one we see all around us.
 His emphasis.
 I refer any offended etymologists to the comment of Humpty Dumpty in Through the Looking Glass: “When I use a word,’ Humpty Dumpty said, in a rather scornful tone, ‘it means just what I choose it to mean, neither more nor less.’”
Adapted from my book In Search of the Multiverse (Penguin)