1 A program that processes a sequential stream of text, carrying out some simple transformation, e.g. condensing multiple spaces to single spaces, counting words, etc. Powerful effects can be created by connecting a series of filters in a pipeline, where each filter takes as its input the output produced by its predecessor.
2 A simple electric circuit or some more complicated device used in the process of filtering. See band-pass filter, low-pass filter, high-pass filter.
3 A set of conditions applied to data so that only certain conforming items are displayed or processed.