Indistinct but not blurred vision with a propensity to headaches, tired and watery eyes and a possible likleyhood of migraines.
Created by a non spherical shape of the cornea creating a different focus power through vertical or horizontal slices of it, or obliquely but always at 90 degrees to each other. Simplest way is to think of a non spherical shape like a rugby ball. If it were transparent it would focus light differently through its different curvatures. This can result in grater degrees of long sight or short sight for each axis or mixed astigmatism where one axis will be long sighted and the other short sighted.
The placement of an astigmatic lens exactly to coincide within a few degrees of the corneal deformity. Often when first prescribed this will require the brain to understand that continual focussing between each plane is not required and so for the first few days will create a tilting or nauseas effect which will soon disappear.