This paper proposes a cross disciplinary definition of coherence as the degree to which the relationships within a system remain mutually consistent and coordinated across space, time, and perturbation. It tests the definition across physics, neuroscience, biology, systems science, music, probability, epistemology, and linguistics while distinguishing coherence from synchronization, correlation, order, stability, and function. The paper also distinguishes coherence, dissonance, and decoupling and establishes a conceptual foundation for quantitative work including Coherence Density.