in mathematics, a generalization and abstraction of the notion of a curved surface; a manifold is a topological space that is modeled closely on Euclidean space locally but may vary widely in global properties. Each manifold is equipped with a family of local coordinate systems that are related to each other by coordinate transformations belonging to a specified class. Manifolds occur in algebraic and differential geometry, differential equations, classical dynamics, and relativity. They are studied for their global properties by the methods of analysis and algebraic topology, and they form natural domains for the global analysis of differential equations, particularly equations that arise in the calculus of variations. In mechanics they arise as “phase spaces”; in relativity, as models for the physical universe; and in string theory, as one- or two-dimensional membranes and higher-dimensional “branes.”