This is similar in principle to roughness except it quantifies subjective perception of slower, up to 20Hz, amplitude modulation of a sound.
The unit of measure for fluctuation strength is the vacil. One vacil is defined as the fluctuation strength produced by a 1000Hz tone of 60dB which is 100% amplitude modulated at 4Hz. Maximal values are found to occur at a modulation frequency of 4 Hz.
The following relation given is from Zwicker E., Fastl H. "Psychoacoustics: Facts and Models" (1990):
F = fluctuation strength [vacil]
fmod = modulation frequency [Hz]
ΔL = masking depth