A variable is a non-numerical symbol used in a mathematical expression that serves as a placeholder for any of a specified set of values. A large part of elementary algebra is devoted to learning techniques for determining the value (or set of possible values) of a variable that appears in an equation or inequality.
- e — typically representative of Euler's number.
- π — pi, which has multiple mathematical meanings but is most commonly used to represent the ratio of a circle's circumference to its diameter (approximately 3.14159).
- o — usually avoided by mathematicians for risk of confusion with the empty set (∅) or the numeral zero (0). (But see Order notation, Big O notation and Little O notation.)
- f — the prototypical name of a function.