The Collaborative International Dictionary of English v.0.48:

Filter \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.
   [1913 Webster]

   Filtering paper, or Filter paper, a porous unsized paper,
      for filtering.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Filter \Fil"ter\, v. i.
   To pass through a filter; to percolate.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Filter \Fil"ter\, n.
   Same as Philter.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Filter \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.
   [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

filter
    n 1: device that removes something from whatever passes through
         it
    2: an electrical device that alters the frequency spectrum of
       signals passing through it
    v 1: remove by passing through a filter; "filter out the
         impurities" [syn: filter, filtrate, strain, separate
         out, filter out]
    2: pass through; "Water permeates sand easily" [syn:
       percolate, sink in, permeate, filter]
    3: run or flow slowly, as in drops or in an unsteady stream;
       "water trickled onto the lawn from the broken hose"; "reports
       began to dribble in" [syn: trickle, dribble, filter]

Moby Thesaurus II by Grady Ward, 1.0:

87 Moby Thesaurus words for "filter":
   bleed, bolt, butterfly, cheesecloth, clarifier, clarify, clean,
   clear, cloth, colander, condense, cradle, cribble, decrassify,
   depurate, diffusing screen, discharge, distill, drain, dribble,
   drip, dripple, drop, edulcorate, effuse, eliminate, elute, emit,
   essentialize, exclude, excrete, exfiltrate, extract, extravasate,
   exudate, exude, filtrate, frosted glass, frosted lens, gauze,
   gelatin filter, give off, ground glass, gurgle, leach, lens hood,
   light filter, lixiviate, lixiviator, membrane, ooze, pass through,
   percolate, percolator, purifier, purify, rectify, reek, refine,
   refiner, refinery, riddle, rocker, run through, screen, seep,
   separate, sieve, sift, sifter, smoked glass, spiritualize, spurtle,
   stained glass, strain, strainer, sublimate, sublime, transude,
   trickle, try, weed out, weep, winnow, winnowing basket,
   winnowing fan, winnowing machine



The Jargon File (version 4.4.7, 29 Dec 2003):

filter
 n.

    [very common; orig. Unix] 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 Free On-line Dictionary of Computing (30 December 2018):

filter

   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]