a phase
Picture a glass of iced water. The ice is one thing, the liquid water is another, the air above is a third. Each is uniform inside itself, and you can point to a boundary where one stops and the next begins. Each of those uniform, separable portions is a phase. A phase is any region of a system that is the same all the way through — same crystal structure (or same liquid or gas), same composition — and is physically distinct from its neighbours across a surface you could, in principle, separate.
The key words are structure and composition. Two chunks of the very same chemical can still be two different phases if their atoms are stacked differently: quartz and cristobalite are both pure SiO2, yet they are separate crystalline phases because the SiO4 tetrahedra are arranged in different patterns. A liquid and its own crystal are two phases. Even scattered gas-filled pores inside a fired pot count as a phase (the vapour). A fired porcelain is a good example of many phases at once: needle-like mullite crystals, leftover quartz grains, a continuous glassy phase gluing them together, and porosity — four phases in one solid.
Why a ceramist cares: nearly every property you can name is decided by which phases are present, how much of each, and how they are arranged. The glassy phase along the grain boundaries governs high-temperature creep and dielectric loss; a second crystalline phase can toughen or embrittle; a trace of liquid that appears on firing is what welds a body together. Counting and identifying phases is the first thing a phase diagram lets you do, and the Gibbs phase rule (F = C - P + 2) is literally a formula about how many phases can coexist.
Hold a phase diagram at one point — say pure alumina held at 1600 degrees C. Read straight off the map: one phase, solid corundum. Now heat pure ice at 0 degrees C and 1 atmosphere: ice and liquid water coexist — two phases at once. The diagram tells you the phase count before you ever touch the material.
A phase is a uniform, separable piece; a diagram's regions each name the phase (or phases) stable there.
Same chemistry does not mean same phase: quartz, tridymite and cristobalite are all SiO2 but are three distinct crystalline phases. Conversely, a solid solution spanning a range of compositions can still be a single phase, because it keeps one continuous structure.