Etymology
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variable (n.)
"quantity that can vary in value," 1816, from variable (adj.) in mathematical sense of "quantitatively indeterminate" (1710). Related: Variably; variability.
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variable (adj.)
late 14c., of persons, "apt to change, fickle," from Old French variable "various, changeable, fickle," from Late Latin variabilis "changeable," from variare "to change" (see vary). Of weather, seasons, etc., attested from late 15c.; of stars, from 1788.
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invariable (adj.)

"constant, uniform, unchanging," early 15c., from Old French invariable (14c.) and directly from Medieval Latin invariabilis, from in- "not, opposite of" (see in- (1)) + variabilis "changeable" (see variable). Related: Invariably.

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exponential (adj.)

"of or pertaining to an exponent or exponents, involving variable exponents," 1704, from exponent + -ial. As a noun in mathematics from 1784. Related: Exponentially.

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vector (n.)

"quantity having magnitude and direction," 1846; earlier "line joining a fixed point and a variable point," 1704, from Latin vector "one who carries or conveys, carrier" (also "one who rides"), agent noun from past-participle stem of vehere "carry, convey" (from PIE root *wegh- "to go, move, transport in a vehicle"). Related: Vectorial.

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inconstant (adj.)
c. 1400, "fickle, not steadfast," from Old French inconstant "variable, eccentric" (14c.), from Latin inconstantem (nominative inconstans) "changeable, fickle, capricious," from in- "not, opposite of, without" (see in- (1)) + constantem (see constant). Related: Inconstantly.
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differential (adj.)
Origin and meaning of differential

1640s, "making or exhibiting a difference," from Medieval Latin differentialis, from Latin differentia "diversity, difference" (see difference). Related: Differentially. As a noun in mathematics, "an infinitesimal difference between two values of variable quantity," from 1704. Differential calculus is attested from 1702.

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Algol 
variable star (Beta Persei) in the constellation Perseus, late 14c., literally "the Demon," from Arabic al-ghul "the demon" (see ghoul). It corresponds, in modern representations of the constellation, to the gorgon's head Perseus holds, but probably it was so called because it visibly varies in brightness every three days, which sets it apart from other bright stars.

The computer language (1959) is a contraction of algo(rithmic) l(anguage); see algorithm.
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irregular (adj.)
late 14c., "not in conformity with Church rules," from Old French irreguler "irregular, incapable, incompetent" (13c., Modern French irrégulier), from Medieval Latin irregularis "not regular," from assimilated form of in- "not, opposite of" (see in- (1)) + Latin regularis "having rules" (see regular (adj.)). General sense of "not conforming to regular rules or principles" is from late 15c. "It expresses the fact of being out of conformity with rule, but implies nothing more with certainty. Yet the word is sometimes used in a sinister sense, as though it were a euphemism for something worse." [Century Dictionary] Meaning "unsymmetrical" is from 1580s. In reference to variable stars, from 1797.
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