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]

WordNet (r) 3.0 (2006):

filter paper
    n 1: a porous unsized paper used for filtering