A phase is a bit of matter characterized by uniform composition and physical structure.
So in addition to the three "phases" of matter that we learn about in elementary school, here we will use phase in a different way. So we can have two coexisting liquid phases, two solid phases, etc.
A phase need not be continuous. For example, an emulsion has two phases only one of which is continuous, while the other is "dispersed".
When we have two coexisting phases, they are separated by a phase boundary.
Phase equilibrium characterizes a system that has multiple coexisting phases whose individual phases (and their amounts) do not change and have no tendency to change on a macroscopic scale.
Note that there will be microscopic transport across the phase boundary, but it will be balanced.
This means, for example, that the number of molecules that condense would be balanced by the number that vaporize.
The phases must also be in mechanical and thermal equilibrium.
Define the term phase and explain what it means for phases to be in equilibrium