Algebraically closed field
A field F is said to be algebraically closed, if every polynomial of degree at least 1 has a zero in F.
As an example, the field of real numbers is not algebraically closed, because the polynomial x2 + 1 has no real zero.
By contrast, the field of complex numbers is algebraically closed, which is the content of the Fundamental Theorem of Algebra.
Every field which is not algebraically closed can be formally extended by adjoining roots of polynomials without zeros.
If one adjoins to F all roots of all polynomials, the resulting field is called the algebraic closure of F.