Barrel distortion
From The Glossary of Digital Photography
A lens aberration in which straight lines in the image bend outward, creating a barrel-like look. This is most common in very-wide-angle lenses and at the widest settings of zoom lenses. It is most noticeable with straight lines within the image and especially with straight lines near the edge of the image. A fish-eye lens is an extreme example of barrel distortion. Some software tools, such as Photoshop, will help correct barrel distortion. See also aberration
