Refers to the frequency domain in the context of [cepstrum]. This is all transformations on signals in different domains after transitioning them between them.
As far as I can tell, you take a signal and FFT it to get time domain representation (spectrum), then take the logarithm of the magnitudes of each of the buckets, and then somehow map that into the "quefrency domain".
"The quefrency is a measure of time, though not in the sense of a signal in the time domain" - https://en.wikipedia.org/wiki/Cepstrum