Aliasing is the misrepresentation of high frequencies as lower frequencies. An alias appears in sampled data acquired at too low a sampling rate compared to the Nyquist frequency of the signal being sampled.

In systems where you want to perform accurate measurements using sampled data, the sampling rate must be set high enough to prevent aliasing, or an optional anti-aliasing filter must be introduced before the A/D converter to restrict the bandwidth of the input signal to meet the sampling criteria. An anti-aliasing filter attenuates unwanted high-frequency signals (which otherwise would appear as undesired, aliased frequency components) of an analog signal prior to its conversion into a digital value. The actual bandwidth in which you can make correct measurements without aliasing is called the alias-free bandwidth. After aliasing has been introduced into a sampled signal, there is no general way to remove it.

In video applications, several forms of visual aliasing are possible. Temporal aliasing, where the wheel of a car seems to turn backwards, raster scan aliasing that results in twinkling on sharp horizontal lines, and stair-step aliasing, where jagged edges appear on lines running at an angle to the scan.