Hello Astigmatism is a type of refractive error wherein you need cylindrical powered glasses to see things clear

A sphere(circle) has 360 degrees and when the curvature of a sphere is equal at all degrees ,it is known as a sphere and number we call spherical. But when the curvature of the sphere is more or less at a particular angle or degree then it's known as  astigmatsm and the number is called cylindrical number at a particular axis(angle)