Nomenclature
When Edward Witten named M-theory, he did not specify what the M stood for—perhaps because the nascent theory was not fully defined. Some, including Sheldon Glashow, speculate that Witten chose the letter because it resembles an inverted W. According to Witten, "M can stand variously for 'magic', 'mystery', or 'matrix', according to one's taste."
Faced with this ambiguous initial, countless scientists and commentators have offered their own expansions of the M—some sincere, others facetious. M should stand for membrane, say some. Meanwhile, Michio Kaku, Michael Duff, Neil Turok, and others suggest mother or master (i.e., the "mother of all theories" or the "master theory").
Although Witten coined the term M-theory to refer to his model of an eleven-dimensional universe, other scientists have generalized the moniker for application to any of various meta-theories involving string theory and brane cosmology. (Ashoke Sen proposed u-theory (ur, 'über', 'ultimate', 'underlying', or perhaps 'unified') as a more distinctive appellation.) When unqualified, M-theory now usually denotes this more general definition, rather than the one Witten originally advanced.
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