One could say vapor pressure, again. When you have a solid and a liquid phase of a given material, each of these has its own equilibrium with the vapor. The favored phase at a given temperature and total applied pressure is the one with lower vapor pressure, since the other phase would "evaporate" enough to then favor condensation of the lower vapor-pressure phase. The freezing point is then the temperature where the vapor pressures are equal allowing both condensed phases to be in equilibrium with thecsame vapor.

One could say vapor pressure, again. When you have a solid and a liquid phase of a given material, each of these has its own equilibrium with the vapor. The favored phase at a given temperature and total applied pressure is the one with lower vapor pressure, since the other phase would "evaporate" enough to then favor condensation of the lower vapor-pressure phase. The freezing point is then the temperature where the vapor pressures are equal allowing both condensed phases to be in equilibrium with the same vapor.