field level

Logarithm of the ratio of two values of a field quantity, usually amplitude. Levels are expressed in different ways: $L F = ln ( F F 0 ) N p$ , where $N p$ is the symbol for the unit neper coherent with the SI, or $L F = 20 log 10 ( F F 0 ) dB$ , where $dB$ is the symbol for the unit decibel.

Source:

Green Book, 2nd ed., p. 79