Source : Webster's Revised Unabridged Dictionary (1913)

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.

   {Filtering paper}, or {Filter paper}, a porous unsized paper,
      for filtering.

Source : WordNet®

filter paper
     n : a porous unsized paper used for filtering