ASME Y14.5 : Dimensioning and Tolerancing
The condition of a surface in which (a) for a feature other than a sphere, all points of each circumferential line created by the surface intersected by any plane perpendicular to the axis or spine (curved line) are equidistant from that axis or spine. (b) for a sphere, all points of the surface intersected by any plane passing through a common center are equidistant from that center.
See Also:
Surface, Feature