critical point
Heat water in a sealed, strong container and something strange happens. The boundary between liquid and steam, normally so obvious, starts to blur, the liquid swells and the gas thickens, until at one exact temperature and pressure the two become indistinguishable — you can no longer say where the water ends and the vapor begins. That exact knife-edge condition is a critical point.
A critical point is the special set of conditions at which a continuous phase transition occurs, the place where the difference between two phases shrinks to nothing. At that point the order parameter just barely reaches zero, the material's response to tiny pushes becomes enormous, and fluctuations of every length scale appear at once. Sit at the critical point and the system looks the same whether you examine it up close or far away — a kind of self-similar shimmer that physicists call scale invariance.
Critical points matter because they are where the deepest and most universal laws of phase transitions reveal themselves, the same mathematics ruling magnets, fluids and alloys alike. A common misconception is that a critical point is just 'the boiling point but special'. It is more profound: it is the unique spot where the very distinction between two phases dissolves, and matter teeters between order and disorder.
Near the critical point of carbon dioxide, around 31°C under high pressure, the once-clear fluid turns milky and glowing — a phenomenon called critical opalescence. It happens because density fluctuations grow to the size of the wavelength of light, so the fluid scatters light in every direction at once, like fog made of the substance arguing with itself.
Critical opalescence: a clear fluid turns cloudy at its critical point as fluctuations grow to the scale of light.
The word 'critical' here means 'pivotal' or 'at a turning point', not 'dangerous'. The critical point is the single condition where a continuous transition lives; the broader cluster of strange behaviors found around it goes by the name critical phenomena.