The foundation of algebra is the **variable**, an unknown number represented by a symbol (usually a letter such as *x* or *a*). Variables can be preceded by a **coefficient**, which is a constant in front of the variable, such as 4*x* or −2*a*.

An **algebraic expression** is any sum, difference, product, or quotient of variables and numbers (such as 2*x* + 7*y *− 1). The value of an expression is found by replacing the variable with a given value and simplifying the result.