A manifold is a topological space locally homeomorphic to an open ball in some Euclidean space. Informally, this says that if one were living on a point in a manifold, the region surrounding any point would look just like "normal" Euclidean space, i.e. for some . For example, the interior of a Mobius strip (that is, excluding its edge) or the surface of an infinite cylinder is a two-dimensional manifold because from each point on either surface the immediate neighborhood is topologically the same as the usual Euclidean plane, even though globally neither of these surfaces looks much like the plane.
Note that the above describes a manifold in the topological category; in the smooth (analytic, holomorphic, etc) category, one would require the patching homeomorphisms to in fact be (analytic, holomorphic, etc).
There are also the generalizations of a manifold with boundary, a manifold with corners, and manifolds with even more funky singular points.
A topological space is said to be a manifold if and only if
- is Hausdorff
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