spherical aberration

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The blurring of an image that occurs when light from the margin of a lens or mirror with a spherical surface comes to a shorter focus than light from the central portion.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun optics A type of lens aberration which causes blurriness, particularly away from the centre of the lens.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an optical aberration resulting in a distorted image