A phase is a uniform region of a material with a single component. Every material has within it some number of regions that appear homogeneous; the count of those regions is the value of the material's phase_of_matter property.
A phase has nothing to do with solids, liquids, or gases. That is the first thing to set straight. The everyday use of the word "phase" gestures at states of aggregation, but here the term is used precisely. A phase is a region. The question we ask of any material is, can you walk through it, sample it, and find that the property values are the same in every place you check? Each region for which the answer is yes is one phase. The number of such regions is what we mean by the value of phase_of_matter.
A phase has features of its own; the named, valued descriptions of those features are its properties. The core features of a phase include temperature, pressure, amount of substance, volume, mass, energy, entropy, chemical potential, momentum, and electric charge. Each of these has a value within the region. That values are uniform across the region is what makes it one phase rather than several.
The phase concept does the heavy lifting in classifying materials. A substance is a material whose phase_of_matter value is one. A mixture is a material whose phase_of_matter value is greater than one. A solution is a material whose phase_of_matter value depends on composition. Three classes, one underlying property, one underlying idea: how many uniform regions are there inside this material? Answer that, and most of the classification work is done.
Question
A glass of ice water has clear cubes floating in clear liquid. How many phases are there, and does it matter that both look like the same kind of stuff?
Reveal Merrill's answer →Hide answer
Two phases. The ice is a uniform region with one set of property values; the surrounding water is another uniform region with a different set. That they are made of the same component does not collapse them into one region; the test for a phase is uniformity of property values within a region, not the identity of the component. A phase has nothing to do with whether something is a solid, a liquid, or a gas. It has to do with how many regions of uniform properties a material contains.