A phase diagram displaying spinodal curves, within the coexistence curves and two critical points, a UCST and an LCST.
In thermodynamics, the spinodal is the limit of stability of a solution, denoting the boundary of absolute instability of a solution to decomposition into multiple phases. Within this curve, infinitesimally small fluctuations in composition and density will lead to phase separation via spinodal decomposition. Outside of the curve, the solution will be at least metastable with respect to fluctuations. In other words, outside of the spinodal curve some careful process may at least in theory obtain a single phase system. Inside of it, no process will. In general, the spinodal lies inside of a binodal (coexistence) curve, which denotes the minimum-energy equilibrium state of the system.