Critical Point
Definition
The temperature and pressure above which liquid and gas become indistinguishable — a single ‘supercritical fluid.’
Technical definition
The end of the liquid-gas coexistence curve on the phase diagram, defined by critical temperature T_c and critical pressure P_c. Beyond it, no amount of compression liquefies the gas; the fluid has liquid-like density with gas-like diffusivity.
Example
Water's critical point is 374°C at 218 atm. Supercritical CO₂ (above 31°C, 74 atm) is used to decaffeinate coffee — it dissolves caffeine like a liquid but flows like a gas.
Common confusion
Above the critical temperature, ‘liquid’ and ‘gas’ stop being meaningful categories — there is just one supercritical phase. It is not ‘very hot gas’; it is a distinct state with properties of both.