Unable to be factorized. So prime numbers are irreducible integers, and x² + 1 is irreducible when considered as a real polynomial though it is reducible to (x + i) (x−i) as a complex polynomial where $i=\sqrt{-1}$. More generally, an element x of an integral domain R is irreducible if x is non-zero, not a unit, and whenever x = yz where y, z ∈ R, then y or z is a unit. In a unique factorization domain an element is irreducible if and only if it is a prime element, but this is not the case generally in rings.