Néron–Severi group

《Algebraic Geometry》 · 第 140 页 · Chapter II，Remark 6.10.3

If $X$ is a nonsingular projective variety of dimension $\geq 2$, then one can define a subgroup $\operatorname{Cl}^0X$ of $\operatorname{Cl}X$, namely the subgroup of divisor classes algebraically equivalent to zero, such that $\operatorname{Cl}X/\operatorname{Cl}^0X$ is a finitely generated abelian group, called the Neron-Severi group of $X$.