It probably stands for "degree". In a tree, the degree of a node is how many children it has. (And before anybody gets pedantic: technically, this is the node'

It probably stands for "degree". In a tree, the degree of a node is how many children it has. (And before anybody gets pedantic: technically, this is the node's "outdegree" when the tree is represented as a directed graph. If it was an undirected graph, we would have to count the parent node as well.)