Definition:Subfield

Let $$\left({F, +, \circ}\right)$$ be a field.

Let $$K$$ be a subset of $$F$$ such that $$\left({K, +, \circ}\right)$$ is also a field.

Then $$\left({K, +, \circ}\right)$$ is a subfield of $$\left({F, +, \circ}\right)$$.