Ingeometric optics,stigmatism refers to the image-formation property of an optical system which focuses a single point source in one phase optics space[clarification needed] into a single point in image space. Two such points are called a stigmatic pair of the optical system. Many optical systems, even those exhibitingoptical aberrations, includingastigmatism, have at least one stigmatic pair. Stigmatism is applicable only in the approximation provided bygeometric optics. In reality, image formation is, at bestdiffraction-limited, and point-like images are not possible due to the wave nature of light.