Distortion caused by sampling, such as digital sampling of audio, or the pixelated sampling a CCD performs on the incoming light. If the sampling rate is too low, the information will be distorted; for example, if you take a digital picture of lines that are spaced less than a two pixels apart, you will not see the original lines. Instead you will see ”ghost” lines spaced farther apart. The same effect makes wagon wheels appear to turn backwards in movies, which are a series of pictures (the samples) updated only 24 times a second.