In thermodynamics, a phase diagram is a graphic representation in two and sometimes three dimensions of the various phases of a substance such as water, an alloy etc.
Usually the thermodynamic temperature and pressure variables are used. In the diagram below each point corresponds to a pair of temperature and pressure conditions. According to these conditions a pure substance will exist in the various liquid, solid etc. phases. Transformations from one phase to another are given precise names. Thus sublimation is the name given for when a solid passes directly to the gaseous state. The term phase transitions is used.
Phase diagram for a pure substance (Credit: University of Scherbrooke).
There are phase diagrams in geology and astrophysics for example. Thus, in precise temperature and density conditions, hadrons become unconfined and form a quark and gluon plasma called a Quark Gluon Plasma or QGP.
This is shown in the diagram below.
QGP phase diagram (Credit: in2p3).
Water phase diagram; for a given temperature and pressure couple in the diagram above the water will be a liquid, a gas or in one of its many ice states.