From Appropedia

A filter is a logical object used to separate two or more substantial kinds, e.g. removing particulates from air or water. A slightly more expansive and general definition includes processes and non-physical objects used to define and differentiate stratum of information, whether that be sieving soil for sand particles or searching for information within a radio signal. Abstractly, a filter of type A removes A in it's discrete components a from some conglomeration S1 so that S2 = S1 - (a1, a2, etc)