What does $[M:N]$ mean, where $M,N$ are modules and $N$ is a submodule of $M$.

Question

What does $[M:N]$ mean? where $M,N$ are modules and $N$ is a submodule of $M$. The context is the following equation: $$\text{disc}(1,\alpha,\alpha^2)=[\mathcal{O}_K:\mathbb{Z}[\alpha]]^2\text{disc}(\mathcal{O}_K)$$ where $\mathbb{Z}[\alpha]$ is the ring of algebraic integers and $\mathbb{Z}[\alpha]$ is a submodule of $\mathbb{Z}[\alpha]$(here $\alpha$ is a algebraic integer).