The technique of minimizing the distortion artifacts when representing a high-resolution image at a lower resolution
In digital signal processing, spatial anti-aliasing is the technique of minimizing the distortion artifacts known as aliasing when representing a high-resolution image at a lower resolution. Anti-aliasing is used in digital photography, computer graphics, digital audio, and many other applications. Anti-aliasing means removing signal components that have a higher frequency than is able to be properly resolved by the recording (or sampling) device. This removal is done before (re)sampling at a lower resolution.
In signal acquisition and audio, anti-aliasing is often done using an analog anti-aliasing filter to remove the out-of-band component of the input signal prior to sampling with an analog-to-digital converter. In digital photography, optical anti-aliasing filters are made of birefringent materials, and smooth the signal in the spatial optical domain. The anti-aliasing filter essentially blurs the image slightly in order to reduce resolution to below the limit of the digital sensor (the larger the pixel pitch, the lower the achievable resolution at the sensor level). See the articles on signal processing and aliasing for more information about the theoretical justifications for anti-aliasing.
(Summarized from http://en.wikipedia.org/wiki/Spatial_anti-aliasing)
