Wavy distortion
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Wavy distortion is mixed distortion, it is a radial image distortion introduced by the lens itself.
It often shows barrel distortion near the center and pincushion distortion along the edges. This type forms a bubble in the image center and is also reffered to as bubble distortion. The opposite variant with pincushion distortion in the center and barrel distortion along the edges exists too, but is rarely seen in real life lenses.
A photograph showing exaggerated wavy distortion (of the bubble variant)