Physics, but not optics. Also, I read and understood the paper. The Wikipedia link seems to agree with me too. Look at the diagram there, and compare it to the diagram for coma [0].
Spherical aberration causes the single point in the center of the object to focus to a different place depending on where the ray hits the lens. This means that the image of that single point is not fully sharp. If you restrict what parts of the lens can be hit by rays from that point (by closing the iris, for example), you reduce the area in the focal plane that the rays hit.
