A liquid-liquid critical point (or LLCP) is the endpoint of a liquid-liquid phase transition line (LLPT); it is a critical point where two types of local structures coexist at the exact ratio of unity. This hypothesis was first developed by H. Eugene Stanley to obtain a quantitative understanding of the huge number of anomalies present in water.
Near an LLCP, there is always a mixture of two alternative local structures. For instance, in supercooled water, two types of local structures exist, a low-density liquid (LDL) and a high-density liquid (HDL) so above the critical pressure, a higher percentage of HDL exists while below the critical pressure a higher percentage of LDL is present. The ratio r = LDL/(LDL + HDL) is determined according to the thermodynamic equilibrium of the system which is often governed by external variables such as pressure and temperature. A discontinuity is present in r when crossing the LLPT, which separates the LDL rich phase from the LDL poor phase. At any point of the LLPT and its associated LLCP the ratio of LDL to HDL is exactly unity.
The LLCP theory can be applied to all liquids that possess the tetrahedral symmetry. The study of LLCPs is an active research area with hundreds of papers having been published, though only a few of these investigations have been experimental since most modern probing techniques are not fast and/or sensitive enough to study them.