In theoretical physics, M-theory is a new limit of string theory in which 11 dimensions of spacetime may be identified. Because the dimensionality exceeds the dimensionality of five superstring theories in 10 dimensions, it was originally believed that the 11-dimensional theory is more fundamental and unifies all string theories (and supersedes them). However, in a more modern understanding, it is another, sixth possible description of physics of the full theory that is still called "string theory." Though a full description of the theory is not yet known, the low-entropy dynamics are known to be supergravity interacting with 2- and 5-dimensional membranes.

This theory is the unique supersymmetrical theory in eleven dimensions, with its low-entropy matter content and interactions fully determined, and can be obtained as the strong coupling limit of type IIa string theory because a new dimension of space emerges as the coupling constant increases.

Drawing on the work of a number of string theorists (including Ashoke Sen, Chris Hull, Paul Townsend, Micheal Duff and John Schwart), Edwrad Witten of the Institute for Advanced Study suggested its existence at a conference at USC in 1995, and used M-theory to explain a number of previously observed dualities, sparking a flurry of new research in string theory called the second superstring revolution.

According to Witten and others, the M in M-theory could stand for master, mathematical, mother, mystery, membrane, magic, or matrix. Witten reluctantly admits the M in M-theory can also stand for murky because the level of understanding of the theory is so primitive. Yet other physicists in jest suggest it is an anagram by flipping the M to a W to stand for Witten. However, originally the letter was taken from membrane, but since Witten was more skeptical about membranes than his colleagues, he just kept the "M". Later, he let the meaning be a matter of taste for the user of the word "M-theory".

In the early 1990s, it was shown that the various superstring theories were related by dualities, which allow physicists to relate the description of an object in one super string theory to the description of a different object in another super string theory. These relationships imply that each of the super string theories is a different aspect of a single underlying theory, proposed by Witten, and named "M-theory".

M-theory is not yet complete; however it can be applied in many situations (usually by exploiting string theoretic dualities). The theory of electromagnetism was also in such a state in the mid-19th century; there were separate theories for electricity and magnetism and, although they were known to be related, the exact relationship was not clear until James Clerk Maxwell published his equations, in his 1864 paper A Dynamical Theory of the Electromagnetic Field. Witten has suggested that a general formulation of M-theory will probably require the development of new mathematical language. However, some scientists have questioned the tangible successes of M-theory given its current incompleteness, and limited predictive power, even after so many years of intense research.

In late 2007, Bagger, Lambert and Gustavsson set off renewed interest in M-theory with the discovery of a candidate Lagrangian description of coincident M2-branes, based on a non-associative generalization of Lie Algebra, Nambu 3 algebra or Filipov 3-algebra. Practitioners hope the Bagger-Lambert Gustavsson action (BLG action) will provide the long-sought microscopic description of M-theory.