Logarithm of the ratio of a power to a reference power. Levels are expressed in different ways: LP = (1/2).ln(P/P0).Np, where Np is the symbol for the unit neper coherent with the SI, or LP = 10.log10(P/P0).dB, where dB is the symbol for the unit decibel.
Source:
Green Book, 2nd ed., p. 79 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)