RMS (or RMSD) — Root-Mean-Square Deviation. In accordance with ROOT terminology (a de facto standard in certain circles, for better of worse) RMS denotes Standard Deviation, and it would be better to call it RMSD. Often, RMS does not involve mean subtraction, but in our usage it does.

RMS is a measure of the width of the distribution, corresponding to Sigma in the case of Gaussian distribution.