See: relative permittivity
Source:
Green Book, 2nd ed., p. 14 (https://dev.goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
See also:
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1105 (https://doi.org/10.1351/pac199466051077)