@wikipedia

A сhemical substance consisting of one or more chemical components which:

• share the same volume (not necessarily continuous, see below)
  
  
• share the same boundary (not necessarily continuous, see below) with other phases
  
  
• move with the same macroscopic velocity

and existing in the state of thermodynamic equilibrium.

The contact between two phases provides the interface for exchanging of chemical components to reach the 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.

The most popular appearance 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 / Thermodynamic equilibrium