filter

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.