Etymology
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phase (n.)

1705, "phase of the moon, particular recurrent appearance presented by the moon (or Mercury or Venus) at a particular time," back-formed as a singular from Modern Latin phases, plural of phasis, from Greek phasis "appearance" (of a star), "phase" (of the moon), from stem of phainein "to show, to make appear" (from PIE root *bha- (1) "to shine").

Latin singular phasis was used in English from 1660 for each of the aspects of the moon. General (non-lunar) sense of "aspect, appearance, stage of development at a particular time" is attested by 1841. Meaning "temporary difficult period" (especially in reference to adolescents) is attested from 1913.

phase (v.)

"to synchronize, adjust the phase of so as to synchronize," 1895, from phase (n.) in the physics sense of "particular stage or point in a recurring sequence of movement or changes" (1861). Earlier as a bad spelling of faze. Meaning "to carry out gradually" is from 1949, hence phase in "introduce gradually" (1954), phase out "take out gradually in planned stages" (1954). Related: Phased; phasing.

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Definitions of phase
1
phase (n.)
any distinct time period in a sequence of events;
Synonyms: stage
phase (n.)
(physical chemistry) a distinct state of matter in a system; matter that is identical in chemical composition and physical state and separated from other material by the phase boundary;
the reaction occurs in the liquid phase of the system
Synonyms: form
phase (n.)
a particular point in the time of a cycle; measured from some arbitrary zero and expressed as an angle;
Synonyms: phase angle
phase (n.)
(astronomy) the particular appearance of a body's state of illumination (especially one of the recurring shapes of the part of Earth's moon that is illuminated by the sun);
the full phase of the moon
2
phase (v.)
arrange in phases or stages;
phase a withdrawal
phase (v.)
adjust so as to be in a synchronized condition;
he phased the intake with the output of the machine
From wordnet.princeton.edu