What Is A Phase?

In chemistry phases can represent the three "states" or temphases as a function of perature and pressure that a compound can achieve just by looking at a phase diagram. All three phases can be attained with water such as ICE, water or liquid ice, and water vapor. In a phase diagram there is a critical point  for water, or a "triple point" where all phases coeoexist. This must be fascinating at the molecular level, too complicated to describe. Carbon dioxide for instance has suh a vertical line for the solid phase so the liquid phase is not seen at all. Sowe see sublimation or two phases. There are different definitions for a phase in chemistry..for instance in the area of kinetics, or the study of the the rates of a reaction you may want to study the effects of missible solvent phases, or mixtures of organic and aqueous solvents at different percentages that the solvent type and and the % to study the effects of two phases that can mix on the reaction.
  Another way to envision phases or use of term is when you are doing an extraction in chemistry. In this case you want two phases, both liquids that DO NOT mix into one another. This is so you can manipulate the two phases independently.

I hope this helps. I may have overshot an easy definition but it all relevant.