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]

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "filtering":
   bolting, clarification, colature, destructive distillation,
   distillation, edulcoration, effusion, elution, elutriation,
   essentialization, excretion, exfiltration, extraction,
   extravasation, exudation, filtration, leaching, lixiviation, ooze,
   oozing, percolating, percolation, purification, refinement,
   riddling, screening, seep, seepage, separation, sieving, sifting,
   spiritualization, straining, sublimation, transudation, weep,
   weeping, winnowing