Advanced Linear Algebra
《Advanced Linear Algebra》 · 第 93 页 · Chapter 3「The Isomorphism Theorems」，定义段
If $U$ is a subspace of $V$, then $\dim(V/U)$ is called the codimension of $U$ in $V$ and is denoted by $\operatorname{codim}(U)$ or $\operatorname{codim}_V(U)$.