Fil·ter n.  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.


 In·fil·tra·tion n.
 1. The act or process of infiltrating, as of water into a porous substance, of a fluid into the cells of an organ or part of the body, or of persons into a hostile organization.
 2. The substance which has entered the pores or cavities of a body.
    Calcareous infiltrations filling the cavities.   --Kirwan.
 Fatty infiltration. Med. See under Fatty.
 Infiltration gallery, a filter gallery.

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