An equation is a statement declaring that two values are equal. It has an equals sign and expressions on the left and right of the equals sign; the expression on the left and the right are equal.

In most equations, one or both sides have variables associated with them. The variable represents an unknown number. Manipulating the equation to come up with the solution to the variable is known as "solving the equation." Algebra covers a variety of types of equations and methods to solve them. For example, algebra covers linear equations of one, two or an indefinite number of variables, quadratic equations and equations of higher orders of power.