Scattering is coherent whenever the phases of the signals arising from different scattering centres are correlated and incoherent whenever these phases are uncorrelated.
Source:
PAC, 1983, 55, 931. 'Definitions, terminology and symbols in colloid and surface chemistry. Part 1.14: Light scattering (Provisional)' on page 932 (https://doi.org/10.1351/pac198355060931)