An unwanted image effect where excessive noise reduction has also smoothed out much of the fine, textural detail in a photo, making it look soft and unnatural. You see this a lot in high-ISO photos from compact cameras with small sensors, where the maker’s noise reduction is often very aggressive in an attempt to hide image noise.
A fine interference pattern sometimes visible when you photograph fine patterns. It happens when these clash with the rectangular grid of pixels on the camera sensor. Actually, you almost never see it – most cameras have anti-aliasing/low pass filters to prevent, and it doesn’t seem to be an issue for those that don’t.
Any unwanted digital flaw in a photo, such as exaggerated sharpening and edge ‘halos’ around objects, banding or ‘posterisation’ due to excessive image manipulation or sensor spots exaggerated by localised contrast or HDR processes.