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Another word for a 
 (infinitely differentiable) Manifold.  A smooth manifold is a Topological Manifold
together with its ``functional structure'' (Bredon 1995) and so differs from a Topological Manifold because the notion
of differentiability exists on it. Every smooth manifold is a Topological Manifold, but not necessarily vice versa. 
(The first nonsmooth Topological Manifold occurs in 4-D.)  In 1959, Milnor showed that a 7-D Hypersphere can be
made into a smooth manifold in 28 ways. 
See also Differentiable Manifold, Hypersphere, Manifold, Topological Manifold
References
Bredon, G. E.  Topology & Geometry.  New York: Springer-Verlag, p. 69, 1995.