In a physical

Physical chemistry

Physical chemistry is the study of macroscopic, atomic, subatomic, and particulate phenomena in chemical systems in terms of physical laws and concepts...

 or geochemical

Geochemistry

The field of geochemistry involves study of the chemical composition of the Earth and other planets, chemical processes and reactions that govern the composition of rocks, water, and soils, and the cycles of matter and energy that transport the Earth's chemical components in time and space, and...

 system, a solvus is a line (binary system) or surface (ternary system) on a phase diagram

Phase diagram

A phase diagram in physical chemistry, engineering, mineralogy, and materials science is a type of chart used to show conditions at which thermodynamically distinct phases can occur at equilibrium...

 which separates a homogeneous solid solution

Solid solution

A solid solution is a solid-state solution of one or more solutes in a solvent. Such a mixture is considered a solution rather than a compound when the crystal structure of the solvent remains unchanged by addition of the solutes, and when the mixture remains in a single homogeneous phase...

 from a field of several phases which may form by exsolution or incongruent melting

Incongruent melting

Incongruent melting occurs when a substance does not melt uniformly and decomposes into another substance. For example, potassium feldspar decomposes to leucite when it melts. The decomposition is not complete, however...

. The line determines a solid solubility limit which changes as a function of temperature.
it is a locus of points on equilibrium diagram
An example is the formation of perthite

Perthite

Perthite is used to describe an intergrowth of two feldspars: a host grain of potassium-rich alkali feldspar includes exsolved lamellae or irregular intergrowths of sodic alkali feldspar . Typically the host grain is orthoclase or microcline, and the lamellae are albite...

 when an alkali feldspar

Feldspar

Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....

is cooled down.it defines limit of solid solubility in an equilibrium diagram