1. A two-dimensional structure at the boundary between two distinct three-dimensional phases that is physically or chemically distinct from the three-dimensional phases. A membrane resists the movement of molecules between the two phases to a greater or lesser extent.

2. A two-dimensional structure postulated as an alternative to unified-field theory and string theory, as a theory of everything. Membrane theory suffers from the difficulty that the procedure of renormalization does not appear to work for it. Membranes, and other extended objects in higher dimensions, occur as solutions of string theory. See also supersymmetry.