phase

Pluralphases

Formsphases, phasing, phased

1. 1
   stage in timenouncountable

   A distinct part of a sequence or cycle that takes place over a period of time.

   The rollout moved into its second phase last month.

   Teething is just one phase in a child's early development.

   UsagePeople go through a phase, often said with a shrug: it's just a phase; projects have the first phase of work.

   Similarstage, step, period, chapter

2. 2
   do graduallyverbtransitive

   To start or stop something gradually over time, bringing it in or taking it out in stages.

   Managers chose to phase in the changes department by department.

   The factory plans to phase out coal use within a decade.

   UsageAlmost always with in or out: phase in new rules, phase out coal; the object can come before or after the particle.

3. 3
   moon shapenouncountable

   One of the recurring appearances of a celestial body's illuminated disk, defined by how much of it is lit or whether it is visible at all.

   Through a small telescope, Venus can be seen going through phases much like the Moon's, from a thin crescent to a nearly full disk.

   Farmers once timed their planting by the phase of the Moon, sowing some crops only while it was growing fuller.

   UsageMost often in the phases of the moon; a body goes through its phases.

   Note

   Astronomy's version of the old "appearance" meaning concerns how much of a body's lit disk can be seen. That amount depends on the changing geometry between the body, its light source and the observer. The phases of the moon is the familiar phrase, but planets go through phases too. The cycle also includes the point where no lit disk shows at all. Source

   Checked 6 Nov 2026 · 5 facts

   • Phase has an underlying sense of appearance. en.wiktionary.org
   • Astronomical phases concern the visible amount and shape of an illuminated disk. aa.usno.navy.mil
   • Phases depend on the relative positions of the body, light source and observer. aa.usno.navy.mil
   • Planets also undergo phases. science.nasa.gov
   • The lunar phase cycle includes a phase with no visible sunlit disk. science.nasa.gov
4. 4
   state of matternouncountable

   A distinct portion of a material system set apart by its chemical makeup, physical state, or crystal structure, with a sharp boundary from neighboring portions.

   Oil and water separate into two distinct phases when you stop stirring.

   At high pressure, carbon can crystallize into a diamond phase.

   Note

   Chemists draw the line by composition and structure, not only by state. A phase is uniform throughout and meets its neighbor at a sharp interface. That makes the term broader than solid, liquid and gas. Oil and water are two liquid phases in one jar, and a single element such as carbon can form different solid phases with different crystal structures. Source

   Checked 6 Nov 2026 · 6 facts

   • Composition and structure can distinguish phases within the same physical state. chem.libretexts.org
   • A phase is internally uniform. chem.libretexts.org
   • Neighboring phases are conventionally described as separated by a sharp interface. nist.gov
   • Phase is a broader distinction than physical state alone. open.edu
   • Oil and water can coexist as two liquid phases. faculty.college.emory.edu
   • Carbon can form solid phases with different crystal structures. open.edu
5. 5
   nouncountable

   A particular aspect or side of something, as it appears to the eye or as it is considered in the mind.

   He knows every phase of the business, from buying stock to keeping the books.

   Her research touches on nearly every phase of the problem, from the chemistry of the soil to the money the farms lose.

   UsageUsually every phase of something: every phase of the business, of the game.

   Note

   Here a phase is an appearance rather than a stage: one of the different faces something shows, as a landscape changes with the light or a business shows another side. The idea of something showing itself, which the word's Greek source carries, runs through several of its other meanings.

6. 6
   wave timingnouncountable or uncountable

   A particular point or portion in a recurring series of changes, such as the motion of a particle in a wave or vibration, often contrasted with the portion on the opposite side of equilibrium.

   When two sound waves are in phase, their peaks line up and reinforce each other.

   A small delay shifted the signal's phase by half a cycle.

   UsageOften in in phase and out of phase, and in compounds such as phase shift and phase difference.

   Note

   Physics uses the word for position within a cycle: where a vibrating particle is in its repeating motion, often expressed as an angle. Waves whose peaks line up reinforce each other, and a shift of half a cycle puts them on opposite sides of equilibrium. The angle ties this sense to the complex-number angle.

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