Algebra
《Algebra》 · 第 159 页 · Chapter III，Section 5「Modules over a ring」，Definition 5.7 后的定义段
Let $R$ be a commutative ring. An $R$-algebra is a ring homomorphism $\alpha:R\to S$ such that $\alpha(R)$ is contained in the center of $S$. An $R$-algebra $S$ is a division algebra if $S$ is a division ring.