In geometric optics, stigmatism refers to the image-formation property of an optical system which focuses a single point source in object space into a single point in image space. Two such points are called a stigmatic pair of the optical system. Many optical systems, even those exhibiting optical aberrations including astigmatism

Astigmatism

An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different foci. If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp focus at two different distances...

, have at least one stigmatic pair. Stigmatism is applicable only in the approximation provided by geometric optics. In reality, image formation is at best diffraction limited and point-like images are not possible.