Quantum Optical Coherence (not to be confused with quantum coherence) is the property of quantum states of the optical fields to factorize quantum correators, such as $g^{(2)}$.
This is well described by Glauber in his Nobel lecture.[1] In particular, there he clarifies why monochromatic fields have been traditionally (classically) associated to coherence, overlooking the quantum state as it was not even conceived, but also finding that monochromatic stationarity fields are those best able to factorize correlators (his Eq. (17)).