Sometimes we say, “that person is going through a phase.” Somethings are the same; somethings are changing. So, if and when that phase ends, we may need to describe that person differently, alter the way we talk about their personality or attitudes (mental state). Might be for the better or not. More or less permanent.
Phase transition is an important term in modern physics, particularly in cosmology related to the Big Bang theory. For example, in Sean Carroll’s book The Big Picture, this term occurs frequently . He defines the term this way:
As systems evolve through time, perhaps in response to changes in their external environment, they can pass from the domain of applicability of one kind of emergent description to a different one— what’s known as a phase transition. Water is the most familiar example. Depending on the temperature and pressure, water can find itself in the form of solid ice, liquid water, or gaseous water vapor. The underlying microscopic description remains the same— molecules of H2O— but the macroscopic properties shift from one “phase” to another. Because of the different conditions, the way that we talk about the water changes: the density, hardness, speed of sound through the medium, and other characteristics of the water can be completely altered, and our vocabulary changes along with them. (You wouldn’t talk about pouring a block of ice, or chipping a cup of liquid water.) — Carroll, Sean (2016-05-10). The Big Picture: On the Origins of Life, Meaning, and the Universe Itself (p. 101). Penguin Publishing Group. Kindle Edition.
He uses a diagram similar to this one:
On Wikipedia, the term is defined as: “The term phase transition is most commonly used to describe transitions between solid, liquid and gaseous states of matter, and, in rare cases, plasma. A phase of a thermodynamic system and the states of matter have uniform physical properties. During a phase transition of a given medium certain properties of the medium change, often discontinuously, as a result of the change of some external condition, such as temperature, pressure, or others. For example, a liquid may become gas upon heating to the boiling point, resulting in an abrupt change in volume. The measurement of the external conditions at which the transformation occurs is termed the phase transition.”
Language is pivotal. A phase transition may introduce novel properties, requiring new vocabulary for the process and to describe the result. Some vocabulary we use for one phase may not apply to another. For example, in our everyday world, the language about water changes for the different phases. Sometimes there’s a blending of vocabulary, like when we say “water vapor” (do we ever say “water gas”). We rely on context (domain of applicability) in everyday life to avoid misunderstanding. And for convenience — typically saying “ice” rather than “frozen water” or “frozen H2O.” H2O transitions involve melting, freezing, vaporization, condensation (and other vocabulary).
Without a deeper understanding that each phase involves the same thing — H2O molecules, we’d approach these states perhaps like the ancient classical elements of earth, water, air (wind), fire, and aether (void, sky).
Atomic theory changed all that. The parallel in modern science is the four states of matter: solid, liquid, gas , plasma — corresponding to different configurations and energy states of imperceptible molecules and atoms.
The transition from one phase to another may be more or less smooth, more or less structured, more or less asymmetrical, and relatively sudden or not. Some phase transitions are reversible. And in physics, these transitions are hot topics, often with unique vocabulary.
Other posts in my blog will rely on understanding phase transition. Particularly since contemporary naturalism uses this concept to propose a better account of the universe than theism, a caring world without God.
 Carroll lists 8 important phase transitions in cosmic history. The concepts of phase transition and emergence are intertwined. “Entirely new physical properties can come into existence as we change phases, such as solidity or transparency or electrical conductivity. Or life, or consciousness. — Carroll, Sean (2016-05-10). The Big Picture: On the Origins of Life, Meaning, and the Universe Itself (p. 102). Penguin Publishing Group. Kindle Edition.
 In fluid dynamics, fluid refers to both liquids and gases.