Subfield
Definition
If \((\mathbb{K}, +, \times)\) is a field and \(\mathbb{F} \subseteq \mathbb{K}\) is also a field using the same definition of \(+\) and \(\times\), then \(\mathbb{F}\) is called a subfield of \(\mathbb{K}\).
If \(\mathbb{F}\) is a subfield of \(\mathbb{K}\) then \(\mathbb{K}\) is called an extension field of \(\mathbb{F}\).