Diagram Illustrating Astigmatism
An illustration depicting an 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.
Encyclopaedia Britannica (11th ed., vol. 28) (New York, NY: The Encyclopaedia Britannica Company, 1910)