A First Course in Modular Forms
《A First Course in Modular Forms》 · 第 12 页 · Section 1.2「Congruence subgroups」，Definition 1.2.1
A subgroup $\Gamma$ of $SL_2(\mathbb Z)$ is a congruence subgroup if $\Gamma(N)\subset\Gamma$ for some positive integer $N$.