A polynomial is an expression of more than two algebraic terms. The X and Y are the variables. A variable is a quantity that may change within the context of a mathematical problem.
What if the given polynomial function is f(x) = 6? What do you call 6, based on your definition "of more than two algebraic terms." Your definition is the common misconception of how the prefix "poly-" is used in Algebra :-)
You're right about x and y, though :-)