In mathematics, particularly in algebra, a field extension is a pair of fields K ⊆ L, {\displaystyle K\subseteq L,} such that the operations of K are those of L restricted to K. In this case, L is an extension field of K and K is a subfield of L. For example, under the usual notions of addition and multiplication, the complex numbers are an extension field of the real numbers; the real numbers are a subfield of the complex numbers. Wikipedia