The imperfect focusing of a lens
or objective mirror
of a telescope
or other optical instrument or device caused by imperfection in the shape of the lens or mirror. For example, as is often the case, an objective mirror might be more closely spherical than parabolic in shape, causing light rays which strike the edge of the mirror to converge at a location which differs from the location at which light rays striking the center of the mirror converge. Thus the light rays striking the mirror converge on a small range of points, rather than at a single point, or more concisely, they fail to focus perfectly.
That, and some might consider the Perfectly spherical cow to be a spherical aberration.