Introduction to Analytic Number Theory
《Introduction to Analytic Number Theory》 · 第 106 页 · Section 5.1「Definition and basic properties of congruences」，Definition
Given integers $a$, $b$, $m$ with $m>0$, we say that $a$ is congruent to $b$ modulo $m$, and we write $a\equiv b\pmod m$, if $m$ divides the difference $a-b$. The number $m$ is called the modulus of the congruence.