Coupling

Coupling is the mutual dependence and interaction between two or more systems, subsystems, or components, such that the state of each influences the state of the others. In systems theory, coupling is what transforms a collection of independent parts into a coherent whole — and what makes that whole unpredictable from the properties of its parts alone. Tight coupling accelerates the propagation of perturbations, turning local failures into global cascades; loose coupling buffers systems from each other, enabling independent evolution at the cost of coordination overhead. The study of coupling is the study of how boundaries dissolve — and why the attempt to draw sharp boundaries around "the system" is usually the first step toward misunderstanding it.