noun, plural: variables
(1) Something that varies or have no fixed quantitative value.
(2) A quantity that assumes a set of values.
(3) A factor that is subject to change
(4) (meteorology) A shifting wind, especially to distinguish from a trade wind.
(1) Likely to deviate or vary, as from the usual type; aberrant; inconstant; changeable.
(2) Capable of changing; alterable.
A variable is a factor or a quantity whose value may vary in contrast to a constant whose value is fixed. A variable may be expressed in symbols whose value may change depending on the values of the other variables. For instance, in the expression: ax2, the variable here is (represented by) x (a is the coefficient).
Variables are used especially in experiments to determine if there is a probability of causal relationship between them. For example, an experiment is designed to see if a newly developed drug is effective in treating ill patients by giving some of them the drug while others the placebo. In this case, the variables are the treatments (i.e. drug or placebo) and their effects on the patients (i.e. recovery).