Given a field extension one can consider the corresponding automorphism group. The main statement of Galois theory is that, when the field extension is Galois, this group is called the Galois group and its subgroups correspond to subextensions of the field extension.
Even more generally there is an analogue of the Galois group in stable homotopy theory. In fact one can define the Galois group of any presentable symmetric monoidal stable (infinity,1)-category, and there is an analogue of the Galois correspondence. In particular one gets a Galois group associated to an E-infinity ring spectrum. One recovers the Galois group of a scheme as the Galois group of its derived category of quasi-coherent sheaves.
consisting just of those automorphisms of whose restriction to is the identity is called Galois group of the field extension .
The just defined Galois group is the one occurring in the classical Galois theory for fields. The analog of the Galois group in Galois theory for schemes is a fundamental group (of a scheme) and is rarely called a ”Galois group”.
The Galois group of the separable closure of is called the absolute Galois group of . In this case we have is equivalent to the fundamental group of the scheme . In particular the notion of fundamental group (of a point of) a topos generalizes that of Galois group. This observation is the starting point and motivating example of Grothendieck's Galois theory and more generally of that of homotopy groups in an (infinity,1)-topos.
For the Galois group in stable homotopy theory, see