Quefrency is the independent variable of a cepstrum, analogous to time in a time-domain function. If the
original function is parameterized by time, quefrency
also has units of time. A peak at quefrency can indicate an echo delay or a spectral spacing with corresponding
frequency .
The term is an anagram-like rearrangement of "frequency" introduced together
with cepstrum by Bogert et al. (1963).
Bogert, B. P.; Healy, M. J. R.; and Tukey, J. W. "The Quefrency Alanysis of Time Series for Echoes: Cepstrum, Pseudo-Autocovariance,
Cross-Cepstrum and Saphe Cracking." In Proceedings of the Symposium on Time
Series Analysis (Ed. M. Rosenblatt). New York: Wiley, pp. 209-243, 1963.
can indicate an echo delay or a spectral spacing with corresponding
frequency 
.
