https://mazziotti.uchicago.edu/qcdictionary/c_terms/coherence/index.html That multiple states can overlap at once leads to the notion of superposition and entanglement. The maintenance of a phase relationship between such states over time is a coherence. Coherences can exist between quantum systems of the same type of object, such as two molecules, or for different kinds of objects, such as light-matter or electron-phonon interactions. Hugo en-us