Critical temperature

A temperature marking a phase transition or the boundary of a phase region under specified conditions. Its precise meaning depends on the system and transition being studied.

Critical point: The endpoint of a liquid–gas coexistence curve, where liquid and gas become indistinguishable. For a fluid, the critical temperature is the temperature coordinate of this endpoint.
Phase transition: A change between states of matter or other thermodynamic phases as external conditions vary. Critical temperature is defined through a phase transition under stated conditions.
Superconducting critical temperature: The temperature below which a material exhibits superconductivity under specified conditions. It is a widely reported critical temperature with direct implications for superconducting devices.
Triple point: The unique conditions at which three phases of a substance coexist in equilibrium. Unlike a critical point, it marks three-phase coexistence rather than the end of a phase boundary.
Order parameter: A measurable quantity that distinguishes phases by describing their symmetry or organization. Its behavior near the transition helps identify and characterize a critical temperature.
Phase diagram: A map showing which phases are stable at different temperatures, pressures, or compositions. It locates critical temperatures relative to other phase boundaries.
Curie temperature: The temperature above which a ferromagnet loses its spontaneous magnetization. It marks a magnetic phase transition and is often called a critical temperature.
Melting point: The temperature at which a solid and liquid coexist in equilibrium at a specified pressure. Ordinary melting temperatures mark first-order boundaries, not necessarily critical endpoints.
Correlation length: A distance over which fluctuations in a system remain statistically related. It grows near continuous transitions, revealing how the system approaches its critical temperature.
Thermodynamic limit: The ideal limit in which a system's size and particle number grow without bound at fixed density. Sharp critical behavior is defined most cleanly in this limit.

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