Multivariate polynomial in the context of "Absolutely irreducible"

In mathematics, a multivariate polynomial defined over the rational numbers is absolutely irreducible if it is irreducible over the complex field. For example, ${\displaystyle x^{2}+y^{2}-1}$ is absolutely irreducible, but while ${\displaystyle x^{2}+y^{2}}$ is irreducible over the integers and the reals, it is reducible over the complex numbers as ${\displaystyle x^{2}+y^{2}=(x+iy)(x-iy),}$ and thus not absolutely irreducible.

More generally, a polynomial defined over a field K is absolutely irreducible if it is irreducible over every algebraic extension of K, and an affine algebraic set defined by equations with coefficients in a field K is absolutely irreducible if it is not the union of two algebraic sets defined by equations in an algebraically closed extension of K. In other words, an absolutely irreducible algebraic set is a synonym of an algebraic variety, which emphasizes that the coefficients of the defining equations may not belong to an algebraically closed field.