The (unknown) x (I suppose that's what you mean) is a variable. It's commonly used to represent any number. For example if you have the equation 3x+2=5, x represents the number which will verify the equation. x can be used with equations, functions etc.
You use variables to stand for numbers in algebra
In algebra, x is the unknown for which you need the value.
In algebra we consider x,y and z are variables.
x2
Variables stand in the place of unknown numbers. For example, in the following equation, one number is unknown: 2+x=5. The x takes the place of the number that is unknown.
Algebra is using variables such as x to solve a problem. One example is x+3=8 in which x equals 5.
Discrete Algebra and Geometry.
Sometimes... some variables can only stand for one thing, like m= slope, but "x" is a variable that can stand for just about anything; slope, axis, and equations.
x+7
x is the unknown number in algebra
3x^2
X + x = x x.x=x