phase / fāz/ • n. 1. a distinct period or stage in a process of change or forming part of something's development: the final phases of the war| [as adj.] phase two of the development is in progress. ∎  a stage in a person's psychological development, esp. a period of temporary unhappiness or difficulty during adolescence or a particular stage during childhood: you are not obsessed, but you are going through a phase. ∎  each of the aspects of the moon or a planet, according to the amount of its illumination, esp. the new moon, the first quarter, the full moon, and the last quarter. ∎  Riding each of the separate events in an eventing competition.2. Zool. a genetic or seasonal variety of an animal's coloration. ∎  a stage in the life cycle or annual cycle of an animal.3. Chem. a distinct and homogeneous form of matter (i.e., a particular solid, liquid, or gas) separated by its surface from other forms.4. Physics the relationship in time between the successive states or cycles of an oscillating or repeating system (such as an alternating electric current or a light or sound wave) and either a fixed reference point or the states or cycles of another system with which it may or may not be in synchrony. • v. [tr.] (usu. be phased) 1. carry out (something) in gradual stages: the work is being phased over a number of years | [as adj.] (phased) a phased withdrawal of troops. ∎  (phase something in/out) introduce into (or withdraw from) use in gradual stages: our armed forces policy was to be phased in over 10 years.2. Physics adjust the phase of (something), esp. so as to synchronize it with something else.PHRASES: in (or out of) phase being or happening in (or out of) synchrony or harmony: the cabling work should be carried out in phase with the building work.ORIGIN: early 19th cent. (denoting each aspect of the moon): from French phase, based on Greek phasis ‘appearance,’ from the base of phainein ‘to show.’

phase
1. An individually distinct and homogeneous part of a system. For example, liquid water and water vapour are each single phases; a mixture of the two constitutes a two-phase system. Similarly minerals crystallizing from a melt form separate phases within it. A ‘phase boundary’ is the line marking the contact between two constituent or liquid phases.

2. A short unit of time, or an episode of development or change, usually within the context of a longer period. The term has been used informally (see INFORMAL) in this sense in many branches of the Earth sciences, e.g. ‘a phase of igneous activity’, or ‘a cold phase’ during a generally warmer period.

phase In physics, a stage or fraction in the cycle of an oscillation, such as the wave motion of light or sound waves. This is usually measured from an arbitrary starting point or compared with another motion of the same frequency. Two waves are said to be ‘in phase’ when their maximum and minimum values happen at the same time. If not, there is a ‘phase difference’, as seen in interference phenomenon. Phase can also refer to any one of the states of matter.

phase Proportion of the illuminated hemisphere of a body in the Solar System (in particular the Moon or an inferior planet) as seen from Earth. The phase of a body changes as the Sun and the Earth change their relative positions. All the phases of the Moon (new, crescent, half, gibbous, and full) can be observed with the naked eye.

phase of a regularly recurring (periodic) quantity. The stage or state of development of the quantity. It can be expressed, in the form of an angle, as the fraction of a cycle of the periodic quantity that has been completed, with respect to a fixed datum point. Two sinusoidally varying quantities of the same frequency can be in phase (reaching corresponding phases at the same time) or out of phase. In the latter case the difference in phase – the phase difference – is usually expressed as an angle.