This study presents a new frequency domain parameter, Parabolic Spectral Parameter (PSP), for the quantification of glottal volume velocity waveforms that have been obtained by inverse filtering acoustic speech pressure signals. PSP is based on fitting a parabolic function to a pitch-synchronously-computed spectrum of the estimated glottal flow. PSP gives a single numerical value that describes how the spectral decay of an obtained glottal flow behaves with respect to theoretical bounds corresponding to maximal and minimal spectral decay. The performance of the new parameter is compared to three commonly used time-based parameters and to one previously developed frequency domain method by analyzing voices produced by different phonation types.