(1) A porous substance through which a liquid or gas is passed to separate it from contained particulate matter or impurities to sterilize. SYN: filtrum. (2) To use or to subject to the action of a filter. (3) In diagnostic or therapeutic radiology, a plate made of one or more metals such as aluminum and copper that, placed in the x- or gamma ray beam, permits passage of a greater proportion of higher-energy radiation and attenuation of lower-energy and less desirable radiation, raising the average energy or hardening the beam. (4) A device used in spectrophotometric analysis to isolate a segment of the spectrum. (5) A mathematical algorithm applied to imaging data for the purpose of enhancing image quality, usually by suppression or enhancement of high spatial frequencies. (6) A passive electronic circuit or device that selectively permits the passage of certain electrical signals. (7) A device placed in the inferior vena cava to prevent pulmonary embolism from low extremity clot. There are many variants. [Mediev. L. filtro, pp. -atus, to strain through felt, fr. filtrum, felt]