A Thermodynamic system in a state of thermodynamic equilibrium, represented by a сhemical substance consisting of one (pure substance) or more (mixture) chemical components which:
Mechanically it behaves like a properly defined Continuum Body.
The contact between two phases provides the interface for exchanging of chemical components to reach a thermodynamic equilibrium specific to a given temperature and pressure and the fluid composition.
The total mass of chemical components in all adjacent phases remain constant over time but at certain temperature and pressure values some phases may disappear (due to migration of their chemical components to the bordering phases).
Phases are not always continuous in space.
For example, gas bubbles are not connected to each other but may move at the same speed which is different to the hosting liquid speed and they do represent a separate phase with an inter-phase contact surface comprised of a sum of all bubbles' surfaces.
A very popular example of phase segregation is water, oil and gas – all are moving at different speeds in porous reservoir or wellbore or pipelines.
See also
Natural Science / Physics / Thermodynamics / Thermodynamic system
[ State of matter ][ Pure substance ] [ Mixtures ][ Fluid Mixtures ]
[ Thermodynamic equilibrium ][ Vapour Liquid Equilibrium (VLE) ]