From AMS Glossary
The ratio of radar reflectivity measured by means of two signals that differ in one attribute, for example, polarization or wavelength.
As applied to polarimetric radar observations, the differential reflectivity is the ratio of the reflectivity observed with transmitted and received signals of horizontal polarization to that observed with signals of vertical polarization. It is commonly represented by the symbol ZDR. The ratio of radar reflectivity measured with two signals of different wavelength is more commonly described as the dual-wavelength ratio.