A variable (pronounced /ˈvɛərɪəbl/) is a symbol that stands for a value that may vary; the term usually occurs in opposition to constant, which is a symbol for a non-varying value, i.e. completely fixed or fixed in the context of use. The concepts of constants and variables are fundamental to all modern mathematics, science, engineering, and computer programming. Much of the basic theory for which we use variables today, such as school geometry and algebra, was developed thousands of years ago, but the use of symbolic formulas and variables is only several hundreds of years old.

Arabic algebra texts such as the Al-Jabr still described mathematics in full text rather than symbolic formulas, but denoted the variable quantity in Arabic word šay' شَيْء = “thing”;. This was taken into Old Spanish with the pronunciation “šei”, which was written xei, and was soon habitually abbreviated to x; and this is still the customary variable name in many fields today. It started the habit of using letters to represent variables in algebra.

Beyond mathematics, “X” has come to represent a generic placeholder variable whose value is unknown or secret, as in project X or mister X. Varying, in the context of mathematical variables, does not mean change, but rather, dependence on the values of other variables in the expression in which the variable occurs, or dependence of the value of the expression on that of the variable. However, in some languages other than English, one distinguishes between "variables" in functions and "unknown quantities" in equations ("incógnita" in Spanish, "inconnue" in French).

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