A phase diagram, which is understood as a graphical representation of the physical states of materials under varied temperature and pressure conditions, is one of the basic concepts employed in high-pressure research. Its general definition refers to the equilibrium state and stability limits of particular phases, which set the stage for its terms of use. In the literature, however, a phase diagram often appears as an umbrella category for any pressure–temperature chart that presents not only equilibrium phases, but also metastable states. The current situation is confusing and may lead to severe misunderstandings. This opinion paper reviews the use of the “phase diagram” term in many aspects of scientific research and suggests some further clarifications. Moreover, this article can serve as a starting point for a discussion on the refined definition of the phase diagram, which is required in view of the paradigm shift driven by recent results obtained using emerging experimental techniques.