The failure of a lens or mirror to bring light rays to the same focus.  When caused by the shape of the lens or mirror, it is called 'spherical Aberration', when caused by the different refrangibility of light of different colours, it is called 'Chromatic Aberration'.  Both the American Gem Society and the Federal Trade Commission specify the use of a 10x corrected eye loupe for grading diamonds and other gemstones.  A corrected loupe is designed to eliminate Aberration, and consists of two or more optical elements.