hole

Because circles are approximated using regular polygons, an interior of a cylinder may be smaller than you may expect. This is especially true when the cylinder is used to accommodate a rod. The cross section of cylinders created here can completely contain a circle of the given radius.

NOTE. There are other reasons that holes may be undersized when using a 3D printer. For more info : http://hydraraptor.blogspot.com/2011/02/polyholes.html