How would you define a polynomial?

Technically, a polynomial is an expression (as opposed to an equation or formula) that includes one or more variables (e.g. x, y and z) and includes only the addition sign, subtraction sign or a variable to a certain power (e.g. x^2).

Most commonly at A Level, however, it refers to an expression as explained only in terms of the variable x. e.g. x^2 + 2x + 6 is a polynomial. However xy - x^2 + y^2 + 5 would also be a polynomial.