The name "variable" is very evocative and it denotes something that is affected by modifications, under defined circumstances. There are types of variables that varies without restraint.

Different types of letters, such that x,y,z,alpha,beta, denote different types of variables.

The power of variable in a polynomial function indicates the type of the curve that describes the function: f(x) = ax^2 + bx + c, denotes a quadratic function, whose curve is a parabola. You may write instead of f(x), the letter y = f(x). The variable y changes with respect to x, which is involved in the relation ax^2 + bx + c.

You may encounter a variable in different types of functions, as the argument of a trigonometric function (f(x) = sin x) or as a power of an exponential function (f(x) = a^x).

A variable is used in place of a number that has not been solved yet. A variable can be a symbol or a letter.