Shimmer

Definition
Fluctuations in amplitude. Like jitter, measures of shimmer serve as an index of vocal stability. Excessive shimmer has been tied to the perception of hoarseness. A mean amplitude difference of 0.7 dB or variation less than about 7% of the mean amplitude may be expected in a sustained phonation produced by a normal speaker.

Links

 * Utrecht Lexicon of Linguistics
 * Audio demonstration