A porous material through which a liquid or gas is passed in order to separate the fluid from suspended particulate matter. A device containing such a material, especially one used to extract impurities from air or water. Any of various electric, electronic, acoustic, or optical devices used to reject signals, vibrations, or radiations of certain frequencies while allowing others to pass. A colored glass or other transparent material used to select the wavelengths of light allowed to reach a photosensitive material. Computer Science A program or routine that blocks access to data that meet a particular criterion: a Web filter that screens out vulgar sites. To pass (a liquid or gas) through a filter. To remove by passing through a filter: filter out impurities. Computer Science To use a filter to block access to (a website or Web content). To pass through or as if through a filter: Light filtered through the blinds. To come or go gradually and in small groups: The audience filtered back into the hall.