Level of a modular form (reviewed)

A level of a modular form $f$ is a positive integer $N$ such that $f$ is a modular form on a subgroup $\Gamma$ of $\operatorname{SL}_2(\mathbb{Z})$ that contains the principal congruence subgroup $\Gamma(N)$.

The least such integer $N$ is sometimes referred to as “the level” of $f$.