Definition:Subfield

Definition
Let $\struct {S, *, \circ}$ be an algebraic structure with $2$ operations.

Let $T$ be a subset of $S$ such that $\struct {T, *, \circ}$ is a field.

Then $\struct {T, *, \circ}$ is a subfield of $\struct {S, *, \circ}$.

The algebraic structure $\struct {S, *, \circ}$ is usually one of two types, as follows:

Subfield of Field
The definition still holds for a field, by dint of the fact that a field is also a ring with unity.