A non-constant polynomialP(x) is irreducible over a set of numbers if it cannot be factored into the product of two non-constant polynomials over the same set of numbers. An irreducible polynomial is also called a prime polynomial.
All polynomials of the form P(x)=ax+b, where a=0, are prime polynomials.
When a polynomial is not prime, it is called a reducible polynomial. Keep in mind that a given polynomial can be prime over one set of numbers but reducible over another.