A surface is closed if it is compact and has no boundary curves.

For example, a sphere is closed since it has no boundary; but a disk is not closed, since it has a circle for a boundary curve.

The term is also used for higher and lower dimensional objects: a circle is a closed curve, since it has no boundary points, but a line segment is not closed since its endpoints are its boundaries.

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11/1/94 -- The Geometry Center