Gini Index

The Gini Index is a measure of Node Impurity.

$$G=\sum _{k=1}^K{\hat{p}}_{mk}\left(1-{\hat{p}}_{mk}\right)$$

We have that $K$ is the number of classes and $\widehat{p_{mk}}$ is the number of training examples from the $m$ th region and $k$ th class.