the critical point

Follow the liquid-vapour boundary on a phase diagram upward — raising both temperature and pressure — and the line does not go on forever. It stops, abruptly, at a special point where the difference between liquid and gas simply vanishes. This is the critical point. Beyond it there is no boiling, no meniscus, no distinction: you can take the fluid continuously from liquid-like to gas-like without ever crossing a phase boundary.

For the liquid-vapour system the critical point is the place where the coexistence curve terminates and the two phases become identical. On an isotherm it is an inflection point of pressure versus volume, defined by (dp/dV)_T = 0 and (d^2p/dV^2)_T = 0 simultaneously, fixing the critical temperature T_c, pressure p_c and volume V_c. The transition exactly at the critical point is continuous (second-order): the order parameter, the density difference between liquid and gas, goes to zero, while the correlation length and the compressibility diverge.

That diverging correlation length makes density fluctuations grow to the scale of visible light, so a fluid at its critical point scatters light and turns milky — critical opalescence. Critical points are the arena of universality: wildly different systems (fluids, magnets) share the same critical exponents. Caveat: while a liquid-gas line ends at a critical point, the solid-liquid (melting) line does not — a crystal and a liquid differ by a broken symmetry, so they cannot become continuously identical.