1. [n]  device that removes something from whatever passes through it
2. [n]  an electrical device that alters the frequency spectrum of signals passing through it
3. [v]  pass through; "Water permeates sand easily"
4. [v]  remove by passing through a filter; "filter out the impurities"
5. [v]  run or flow slowly, or in an unsteady stream; "reports began to dribble in."

1. \Fil"ter\, n. [F. filtre, the same word as feutre felt,
   LL. filtrum, feltrum, felt, fulled wool, this being used for
   straining liquors. See {Feuter}.]
   Any porous substance, as cloth, paper, sand, or charcoal,
   through which water or other liquid may passed to cleanse it
   from the solid or impure matter held in suspension; a chamber
   or device containing such substance; a strainer; also, a
   similar device for purifying air.
   {Filter bed}, a pond, the bottom of which is a filter
      composed of sand gravel.
   {Filter gallery}, an underground gallery or tunnel, alongside
      of a stream, to collect the water that filters through the
      intervening sand and gravel; -- called also {infiltration
      gallery}.
   

2. \Fil"ter\, v. t. [imp. & p. p. {Filtered}; p. pr. & vb.
   n. {Filtering}] [Cf. F. filter. See {Filter}, n., and cf.
   {Filtrate}.]
   To purify or defecate, as water or other liquid, by causing
   it to pass through a filter.
   {Filtering paper}, or {Filter paper}, a porous unsized paper,
      for filtering.
   

3. \Fil"ter\, v. i.
   To pass through a filter; to percolate.
   

4. \Fil"ter\, n.
   Same as {Philter}.
   

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]