What does the pair of vertical lines in empirical entropy formula mean? I am learning from this post. 

$\alpha$ is the ratio of the first subset,

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

according to the context and code of the post, $\left|D\right|$ means the number of samples?


What are the pair of vertical lines called?


Is it the L1 norm symbol? It does not seem to be the absolute value symbol.
$D$ is a set which is further divided into sets $D_{1}$ and $D_{2}$. In this context the vertical lines represent the cardinality of the set i.e. the number of elements in the set. Hence, $\alpha$ represents the ratio of number of samples of $D_{1}$ to $D$.