What's the origin of the word "Matrix" used as either a name or a description of a virtual reality environment? The best known example is of course from the film of the same name, and it's widely reported that the name in this case was taken from Gibson's Neuromancer, but this is predated by the similarly named computer system on Gallifrey in Doctor Who (first appearing in the episode The Deadly Assassin in 1976, several years before Gibson started publishing). Are there any earlier occurrences, or is this the origin of the word?
There are no earlier references given in the "matrix" entry on p. 117 of Brave New Words: The Oxford Dictionary of Science Fiction, I think it's likely the researchers for the Dictionary would have turned up any if they existed. So, there's a very good chance The Deadly Assassin (1976) is the first example of "matrix" being used to mean "cyberspace or virtual reality", although of course the word itself has earlier meanings (it's used in mathematics, and in biology to refer to some kind of source where something grows).
edit: user14111 posted a link to examples of "matrix" on a useful website of science fiction citations, but note that this page describes the website as "the Oxford English Dictionary's science fiction words site", so presumably the editors of the Oxford Dictionary of Science Fiction would have known about those examples and not considered any of the pre-1976 ones to be examples of matrix being used to mean cyberspace/virtual reality.
Having just watched the 1972 science fiction film Solaris, I was surprised to hear this dialog.
There is no Hari. She's dead. You're just a reproduction, a mechanical reproduction. A copy. A matrix.
Later in the film this applies to more than a "person" but actually entire virtual environments. Not quite identical to "The Matrix", but some similarities.