The adjective variable is used to describe something that is liable to change often, such as the weather, interest rates, or your teacher's mood.
If you mention the noun variable to people who are mathphobic, you may give them nightmares because they don't like to think about numbers or values that can change in equations. The adjective form of variable has been around since the late 1300s, applying first to people, and then, in the late 1400s, to weather. The noun form first appeared in Lacroix's Differential and Integral Calculus in 1816, guaranteeing that math students would be solving for the variable x for generations to come.
Definitions of variable
1
n a quantity that can assume any of a set of values
 Synonyms:
 variable quantity
 Types:
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argument
a variable in a logical or mathematical expression whose value determines the dependent variable; if f(x)=y, x is the independent variable

experimental variable, independent variable
(statistics) a variable whose values are independent of changes in the values of other variables

correlate, correlative
either of two or more related or complementary variables

degree of freedom
(statistics) an unrestricted variable in a frequency distribution

dependent variable
(statistics) a variable in a logical or mathematical expression whose value depends on the independent variable

predictor variable
a variable that can be used to predict the value of another variable (as in statistical regression)

infinitesimal
(mathematics) a variable that has zero as its limit

chance variable, random variable, stochastic variable, variant, variate
a variable quantity that is random

scalar
a variable quantity that cannot be resolved into components

tensor
a generalization of the concept of a vector

vector
a variable quantity that can be resolved into components

factor
an independent variable in statistics

cross product, vector product
a vector that is the product of two other vectors

resultant, vector sum
a vector that is the sum of two or more other vectors

radius vector
a line connecting a point in space to the origin of a polar coordinate system

radius vector
a line connecting a satellite to the center of the body around which it is rotating
 Type of:

quantity
the concept that something has a magnitude and can be represented in mathematical expressions by a constant or a variable
n a symbol (like x or y) that is used in mathematical or logical expressions to represent a variable quantity
 Types:

placeholder
a symbol in a logical or mathematical expression that can be replaced by the name of any member of specified set

unknown, unknown quantity
a variable whose values are solutions of an equation
 Type of:

symbol
an arbitrary sign (written or printed) that has acquired a conventional significance
n a star that varies noticeably in brightness
 Synonyms:
 variable star
 Examples:

North Star
the brightest star in Ursa Minor; at the end of the handle of the Little Dipper; the northern axis of the earth points toward it
 Type of:

star
(astronomy) a celestial body of hot gases that radiates energy derived from thermonuclear reactions in the interior
adj liable to or capable of change
“rainfall in the tropics is notoriously
variable”
“variable winds”
“variable expenses”
 Synonyms:

inconsistent
displaying a lack of consistency

inconstant
likely to change frequently often without apparent or cogent reason; variable

changeable, uncertain, unsettled
subject to change

covariant
changing so that interrelations with another variable quantity or set of quantities remain unchanged

multivariate
pertaining to any procedure involving two or more variables

protean
taking on different forms

shifting
continuously varying

variant
exhibiting variation and change

versatile
changeable or inconstant
 Antonyms:

invariable
not liable to or capable of change

consistent
(sometimes followed by `with') in agreement or consistent or reliable

constant
steadfast in purpose or devotion or affection

changeless, constant, invariant, unvarying
unvarying in nature

hardandfast, strict
(of rules) stringently enforced

invariant
unaffected by a designated operation or transformation
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