peak-to-peak
The highest reading in a window minus the lowest.
Peak-to-peak is max x_i - min x_i over a window; 2A for a sine of amplitude A, and far more sensitive than RMS to single impacts.
Examples
A 40 mg sine has a peak-to-peak of 80 mg, the highest reading minus the lowest.
A sharp knock on the fan housing moves the peak-to-peak far more than the RMS.
See also