Phases

Phases are distinct forms or states of a system characterized by stable properties under given conditions. The term is used in many fields, but it is most familiar in the study of matter, where a phase refers to a form of matter with uniform structure and properties. The classical phases are solid, liquid, and gas; under extreme conditions, plasmas are also considered a phase of matter.

Phase transitions occur when conditions such as temperature or pressure change, causing a system to move from one phase to another. These transitions often involve latent heat, and are described by phase diagrams. Common examples include melting, freezing, vaporization, condensation, sublimation, and deposition.

In astronomy, a phase describes the appearance of the Moon or other celestial bodies as seen from Earth. The Moon’s phase cycle is driven by the relative positions of the Sun, Moon, and Earth and lasts about 29.5 days, producing phases such as new moon, first quarter, full moon, and last quarter.

In other disciplines, phase has related but distinct meanings. In wave physics, phase refers to the position of a point in a periodic cycle, determining interference patterns. In project management or product development, a phase is a stage in a lifecycle, such as initiation, planning, execution, and closure. Phase diagrams in materials science map the stability of different phases against variables like temperature and composition and are used to predict material behavior.