Anti-aliasing filter
From The Glossary of Digital Photography
(1) An optical filter installed directly in front of the digital sensor. It slightly lowers the high contrast areas of the image so that the digital sensor will still be able to sample the image. Since it softens the resulting image, the anti-aliasing filter is sometimes also known as a blur filter. (2) A software plug-in used in other software, such as Photoshop. It accomplishes the same thing by slightly blurring the edges.
