does the operator that has a pair of vertical lines to compute cardinality of a set have a name? if yes, what it is?

Question

I am learning this post.

the $|D|$ in formula

$$\alpha=\frac{\left|D_{1}\right|}{\left|D\right|}$$

means the cardinality of the set D.

does this operator have a name? if yes, what it is?

Answer 1

It is also written $\operatorname{card}D$. So it is natural to call the operator card, following the convention of calling the logarithm, cosine, tangent (etc.) functions after their standard abbreviations log, cos, tan, and so on.