From AMS Glossary
The volume in space within which are located the scatterers that contribute to the radar echo arriving at the receiver at a particular instant.
Individual scatterers within the pulse volume contribute to the instantaneous signal and cannot be resolved. The pulse volume is therefore sometimes called the resolution volume. The extent in range of the pulse volume is determined by the convolution of the transmitted pulse with the receiver filter response and the extent in azimuth by the antenna pattern. Ordinarily, the radial extent of the volume is one-half the pulse length and the transverse extent is the 3-dB beamwidth.