The measurement of the differential mode dc output ripple and noise over a 20 MHz bandwidth.
The amplitude of the AC component on the DC output of a power supply usually expressed in millivolts peak-to-peak or RMS. For a linear power supply it is usually the frequency of the AC mains. For a switching power supply, it is usually the switching frequency of the converter stage.
Periodic and Random Deviation - Noise and ripple voltage superimposed on a converter's DC output. Typically specified at full load, it is expressed in peak-to-peak or RMS volts over a given bandwidth.