field level

https://doi.org/10.1351/goldbook.F02361
Logarithm of the ratio of two values of a field quantity, usually amplitude. Levels are expressed in different ways: LF = ln(F/F0).Np, where N p is the symbol for the unit neper coherent with the SI, or LF = 20.log10(F/F0).dB, where dB is the symbol for the unit decibel.
Source:
Green Book, 2nd ed., p. 79 (https://dev.goldbook.iupac.org/files/pdf/green_book_2ed.pdf)