low-pass filter 

https://doi.org/10.1351/goldbook.08268
Optical filter that excludes radiation of wavenumber greater (wavelength lower) than a certain value and passes radiation of wavenumber below (wavelength greater than) this value. Electrical filter that blocks signal with a modulation frequency above a certain cut-on value and passes signals of frequency below this value.
Source:
PAC, 2021, 93, 647. (Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)) on page 666 [Terms] [Paper]