1. (Originally Unix, now also MS-DOS) A progrAM that processes an input data streAM into an output data streAM in some well-defined way, and does no I/O to anywhere else except possibly on error conditions one designed to be used as a stage in a pipeline (see plumbing). Compare sponge. 2. (functional progrAMming) A higher-order function which takes a predicate and a list and returns those elements of the list for which the predicate is true. In Haskell: filter p [] = [] filter p (x:xs) = if p x then x : rest else rest where rest = filter p xs See also filter promotion. [Jargon File]