Suppose $D$ is a set of pairs: $$D=\{(\boldsymbol {x},y)\in N\}$$ where $N$ is another set, $\boldsymbol{x}$ is an $M$ dimensional vector, and $y$ is a scalar. Then what does the notation $|D|$ means ?
Is this perhaps the L2 norm? :
$$|D|=||D||_2$$ for all $\boldsymbol{x}\in N$
I've seen the formula used in Classification and Regression Trees:
