In mathematical functions, the letter "x" is used as a variable, meaning it's used in place of an unknown value or values. The letter "x" is commonly used to represent a variable, a tradition that started with the French mathematician René Descartes in the 17th century.
A variable of unknown value can be demonstrated by the equation 2 + x = 6. In this example, x has the value of 4. Likewise, a variable with a variety of values can be demonstrated by the equation 2 + x = y. The value of y, a dependent variable, changes as the independent variable x changes. If x = 3, then y = 5.