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.

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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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phase-out (n.)

"gradual, planned removal or elimination," 1958, from the verbal phrase (1954; see phase (v.)).

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phasic (adj.)

"of or pertaining to a phase, of the nature of a phase," 1890, see phase (n.) + -ic

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interphase (n.)
in cytology, 1913, from German interphase (1912); see inter- "between" + phase (n.).
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multiphase (adj.)

also multi-phase, "having or producing two or more phases," 1890, from multi- "many" + phase (n.).

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anaphase (n.)
name of a stage of cell division, 1887, coined in German (1884), from Greek ana "back" (see ana-) + phase (n.).
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telophase (n.)
1895 in cytology, from Greek telo-, combining form of telos "the end, fulfillment, completion" (see telos) + phase (n.).
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prophase 

first stage in a nuclear division, 1884, from German prophase (Strasburger, 1884); see pro- + phase (n.). Greek prophasis meant "that which appears, a motive or pretext."

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*bha- (1)

*bhā-, Proto-Indo-European root meaning "to shine."

It forms all or part of: aphotic; bandolier; banner; banneret; beacon; beckon; buoy; diaphanous; emphasis; epiphany; fantasia; fantasy; hierophant; pant (v.); -phane; phanero-; phantasm; phantasmagoria; phantom; phase; phene; phenetic; pheno-; phenology; phenomenon; phenyl; photic; photo-; photocopy; photogenic; photograph; photon; photosynthesis; phosphorus; phaeton; sycophant; theophany; tiffany; tryptophan.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit bhati "shines, glitters;" Greek phainein "bring to light, make appear," phantazein "make visible, display;" Old Irish ban "white, light, ray of light."

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