physical substance consisting of one or more chemical components whichThermodynamic system in a state of thermodynamic equilibrium, represented by a сhemical substance consisting of one (pure substance) or more (mixture) chemical components which:

• share the same space volume (not necessarily continuous, see

• below)
  
  
• share the same boundary (not necessarily continuous, see

• below) with other phases
  
  
• move with the same macroscopic velocity

• .

Mechanically it behaves like a properly defined Continuum Body.

A Phase can be represented by a Pure chemical substance or by a Mixtureand existing in chemical, thermal and hydrodynamic equilibrium.

The contact between two phases provides the interface for exchanging of chemical components to reach the  of each phase to reach a thermodynamic equilibrium specific to the given a given temperature,  and pressure and the total mass of the components in all phasesfluid composition.

The total mass of chemical components in  in all adjacent phasesremain constant over time but at certain certain temperature and  and pressure values some phases may disappear (due to migration of their chemical components to  to the bordering phases).

Phases are not always continuous in space.

For example, gas bubbles  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 a an inter-phase contact surface comprised of a sum of all bubbles' surfaces.
The most
A very popular appearance example of phase segregation is water, oil, gas   and gas – all are moving at different speeds in porous media reservoir or wellbore or pipelines.