Phase - Thermodynamics:
A phase is quantity of matter that is homogeneous throughout in chemical composition and physical structure.
If the matter is all gas, all liquid or all solid, physical uniformity is there. Likewise, if chemical composition does not vary from one part of system to the other, it has chemical uniformity.
Examples of one phase system are single gas, a single liquid, a mixture of gases or solution of liquid contained in vessel.
A system consisting of liquid and gas is the two phase system.
Water at triple point exists as water, ice and steam at the same time form a 3 phase system.