A variable is a symbol (usually a letter) in mathematical expressions and equations.

A variable is used to represent an unspecified element of a set. Any member of the set is said to be a value of the variable, and the set itself is the domain (or range) of the variable.

• Expression: 2x + 3 [the variable is x]
• Conditional Equation: x + 3 = 5 [the variable is x]
• Identity: x + x = 2x [the variable is x]
• Relation: x + y = 4 [the variables are x and y]

When working with conditional equations, the variable is sometimes called the "unknown".