attenuated total reflection 

Synonym: frustrated total internal reflection
Initialism: ATR
https://doi.org/10.1351/goldbook.08227
Internal reflection from an absorbing material at angles of incidence at or above the critical angle.
See also: multiple attenuated total reflection
Source:
PAC, 2021, 93, 647. 'Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)' on page 652 (https://doi.org/10.1515/pac-2019-0203)