residual ripple
The ripple is defined as the percentage residual component of an AC voltage superimposed on the rectified and smoothed DC voltage.
The ripple is a dimensionless value which is calculated from the quotient of the superimposed residual voltage to the DC voltage. The ripple is given as a percentage.
Rectification of a sinusoidal AC voltage produces a semi-sinusoidal AC voltage. To make a DC voltage out of it, this voltage is smoothed with filter elements. Regardless of the effort of the filter circuit, a residual AC voltage component always remains on the smoothed DC voltage. This superimposed AC voltage is used to determine the residual ripple.