Simple definition of polynomial.
A polynomial is an expression of the form anxn+a(n−1)x(n−1)+a(n−2)x(n−2)+........+a1x+a0,
Where, all ai's belongs to set of real numbers, n is set of all non negative integers and x is called variable or indeterminant.
In other words, an expression contains variables and coefficients, that involves the operation of addition, subtraction, multiplication(except division) and non-negative integer exponentiation of variables is called polynomial.
Example: x2−4x+1 is a polynomial.