According to Witten, M should stand for "magic", "mystery", or "membrane" according to taste, and the true meaning of the title should be decided when a more fundamental formulation of the theory is known.
According to Witten, M should stand for "Magic", "Mystery" or "Membrane", according to taste Duff, MJ. Center for Theoretical Physics, Texas A&M University, 1996, p.2, and the true meaning of the title should be decided when a more fundamental formulation of the theory is known.