When this quantity is coarse-grained, as in the Ginzburg-Landau potential, it's usually just assumed to be the magnetization.
For a double-well, the time derivative of this quantity equals the Laplacian of it plus twice this quantity, times one minus this quantity squared, in a version of the Cahn-Hilliard equation. This quantity is conserved in Class B models in the Hohenberg-Halperin classification for it, while in Class A models it is not. A topological defect is one whose winding number on the field space of this quantity is nonzero, such as (*) hedgehog defects in ferromagnets. This quantity is the same as the wavefunction for a Bose-Einstein condensate, and it equals the difference in density between vapor and liquid at VLE. When this quantity is coarse-grained, as in the Ginzburg-Landau potential, it's usually just assumed to be the magnetization. For 10 points, name this quantity equal to the derivative of free energy with respect to the external field, which Paul Ehrenfest defined to be discontinuous in a first-order phase transition.
For a double-well, the time derivative of this quantity equals the Laplacian of it plus twice this quantity, times one minus this quantity squared, in a version of the Cahn-Hilliard equation.
order parameter [prompt on m; prompt on psi; prompt on order]