critical point
Picture heating a sealed tube of liquid with some vapor above it. The liquid expands and thins; the vapor compresses and thickens. Push the temperature and pressure high enough, and the two grow so alike that the surface between them shimmers, fades, and finally vanishes — you can no longer tell where liquid ends and gas begins. That vanishing act happens at the critical point.
The critical point is the exact temperature and pressure (the critical temperature and critical pressure) at which the distinction between liquid and gas disappears. It marks the end of the liquid–gas boundary line on a phase diagram: the line of coexistence simply terminates there, rather than continuing forever. Above this point, no amount of squeezing will condense the substance back into a recognizable liquid.
Why it matters: beyond the critical point lies a strange fourth regime, the supercritical fluid, which flows like a gas but dissolves like a liquid. The critical point also explains why some gases, like nitrogen or methane, refuse to liquefy at room temperature no matter how hard you compress them — their critical temperature is far below ordinary warmth, so above it, liquid simply cannot exist.
Water's critical point is at 374 °C and 218 atmospheres; carbon dioxide's is far gentler, at 31 °C and 73 atmospheres — which is why CO2 is so easy to turn supercritical for decaffeinating coffee.
Above the critical point, liquid and gas become one indistinguishable fluid.
The critical point terminates the liquid–gas line, but there is no critical point for the solid–liquid line — a solid and a liquid never become indistinguishable, because their molecular order is fundamentally different. So the melting line, unlike the boiling line, runs on without end.