A variable is a symbol, usually a letter, that stands for a number or quantity. In 3x + 5, x is the variable. It can take different values. Variables let us write general expressions and equations that hold for many cases.

In algebra, variables are manipulated to solve equations. In functions, the variable is the input. In statistics, a variable is a characteristic being measured, like height or income. In computer science, a variable is a named storage location.

Variables are essential for modeling. They represent unknowns in word problems, parameters in formulas, and changing quantities in science. The concept of a variable is one of the most powerful abstractions in mathematics.

• Symbol for an unknown or changing quantity
• Usually a letter like x or y
• Used in equations, functions, and statistics
• Essential for general mathematical statements