film at 11 = F = Finagle's Law

filter n.

[very common; orig. Unix, now also in 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.

--The Jargon File version 4.3.1, ed. ESR, autonoded by rescdsk.