Thermodynamic 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

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• 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 Mixture.

The the contact between two phases provides the interface for exchanging the of chemical components to reach the chemical and hydrodynamic equilibrum  of each phase to reach a thermodynamic equilibrium specific to a given given temperature and pressure and pressure.At certain temperature & pressure values a phase may disappear (its chemical components may transfer to the bordering phase).the fluid composition.

The total mass of chemical components in all adjacent phasesremain 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.