The story
A graph of the change in pitch over time of a particular tone resembles a terrace.
Etymology from Wiktionary, CC BY-SA 4.0.
tone terracing
Today it means: A form of downdrift in which the high or mid tones, but not the low tone, shift downward in pitch after certain other tones. The result is that a tone may be realized at a certain pitch over a short stretch of speech, shifts downward and then continues at its new level, then shifts downward again, until the end of the prosodic contour is reached, at which point the pitch resets. Full definition →