How is astigmatism characterized in relation to the cornea?

Astigmatism is primarily characterized by an irregular curvature of the cornea, which leads to a distortion in vision. In a normal eye, the cornea has a smooth, evenly curved shape, allowing light rays to focus clearly on the retina. However, in astigmatism, the cornea may be more steeply curved in one direction than the other, causing light to enter the eye at different angles. This irregular shape results in multiple focal points in front of or behind the retina, leading to blurred or distorted vision at various distances.

The other options suggest conditions that do not accurately describe astigmatism. Excess pressure doesn't directly characterize this refractive error, while a stable surface and uniform shape indicate the presence of a normal cornea, which is contrary to what occurs in astigmatism. Thus, the defining feature of astigmatism lies in the irregularity of the corneal curvature.