A phase transition is a process by which a thermodynamic system undergoes a sudden change in its state or the arrangement of its constituent particles, often marked by changes in volume, density, or magnetic properties. These transitions occur when external parameters, such as temperature or pressure, cross critical points, allowing the system to move from one distinct physical phase to another. Classical examples include solid-liquid transitions (melting) and liquid-gas transitions (boiling). More complex transitions involve changes in internal structural order, such as polymorphism in crystalline materials or shifts in ferroelectric or superconducting states, and are fundamentally governed by minimizing the system's free energy.