An "Order Parameter" is a quantitative measure used in physics and materials science to describe the degree of order in a system. It helps distinguish between different phases, such as solid, liquid, or gas, by indicating how organized the particles are. For example, in a ferromagnet, the order parameter can represent the alignment of magnetic moments. In phase transitions, the order parameter changes value as the system moves from one phase to another. During a transition, such as from liquid to solid, the order parameter increases, reflecting the growing order among particles. This concept is crucial for understanding phenomena like superconductivity and liquid crystals.