Where your image becomes darker the further you get from the center.
This is due to the radial nature of the lens which collects more light in the center. It can be particularly strong with wide angle lenses. Nonunifority is also caused by the chief ray angle of light incident to the sensor which has reduced quantum efficiency as the angle increases.
Light falloff often improves when lenses are stopped down. It can be easily corrected in software or in the image processing pipeline. Picture Window Pro, PTLens, and several other programs have tools for removing it. Because moderate amounts of light falloff can be pictorially pleasing, it’s not always advisable to remove it completely.
Nonuniformity is measured by Uniformity and Uniformity-Interactive. These modules also measure other sensor nonuniformities, including color uniformity, stuck pixels, local sensitivity variations and noise. It has a particularly rich set of displays in Imatest Master and IS.