Arakelov Geometry
《Arakelov Geometry》 · 第 64 页 · Chapter 3，Section 3.1「Orders」
In this book, a commutative ring $R$ is called an order if $R$ is a finitely generated free module as a $\mathbb{Z}$-module. Moreover, an order $R$ is called an integral order if $R$ is an integral domain.