Theorem name

What is the theorem that states if $$\Omega$$ is a polynom with degree > 1 with real coefficients. If there exists a complex number $$z = a + bi$$ such that $$\Omega(a+bi)=0$$ then $$\overline{z} = a - bi$$ is also a root of $$\Omega$$? For $$\Omega(x) = x^2 + px + q$$ with p and q real then if a+bi is a root then a-bi is also a root if $$b \neq 0$$, that one is easy but I don't think it's easy for degree > 2 to prove it that's why I'm search for it's name.