Cepstrum
The spectrum of the logarithm of a spectrum — a way to spot repetition in the frequency domain, such as pitch and echoes, with one extra transform.
Bogert, Healy and Tukey described it in 1963 in a paper on finding echoes in seismic data, and they scrambled the letters of everything they touched: spectrum became cepstrum, frequency became quefrency, filtering became liftering. The recipe is an FFT of the signal, the log of the magnitude, then an inverse transform. The logarithm turns an echo or a fundamental, which multiplies the spectrum by a periodic pattern, into something added to it instead, and it shows up as a single peak at the corresponding delay.
A. Michael Noll used it in 1967 to measure pitch in speech, and the same trick still separates the vocal folds' tone from the mouth's resonances. The low coefficients describe timbre, and those are what MFCC builds on. That the vocabulary has survived sixty years says something about how much fun signal-processing people have.