From Glossary of Meteorology
A radar echo with phase and amplitude either showing little change over successive radar pulses or changing in a regular and predictable way. Such an echo may arise from fixed or slowly moving point targets or from distributed targets in which the individual scattering elements do not move (or move slowly) with respect to one another.
By contrast, an incoherent echo is an echo with random phase and amplitude from pulse to pulse. Such echoes arise from distributed targets such as precipitation targets, in which the individual scatterers move with respect to one another.