Field extension/Definition

Let ${\displaystyle {}L}$ be a field, and let ${\displaystyle {}K\subseteq L}$ be a subfield of ${\displaystyle {}L}$. Then ${\displaystyle {}L}$ is called an extension field of ${\displaystyle {}K}$, and the inclusion ${\displaystyle {}K\subseteq L}$ is called a field extension.