1. Any material that, by absorption or reflection, selectively modifies the radiation transmitted through an optical system. 2. To remove a certain component or components of radiation, usually by means of a filter, although other devices may be used. 3. A mathematical tool used to remove information within certain frequency (or wavelength) limits from a time (or space) series of data. Filters are commonly used to remove high (low) frequency oscillations from a dataset when oscillations of low (high) frequency are of interest.