An internal node in a graph is a node that has degree (number of links to or from other nodes) greater than one. A tree that has more than two nodes can be decomposed into (i) one or more internal nodes, (ii) leaf nodes, and (iii) in case the tree is directed, a root node. Traditionally, in a phylogenetic tree, all extant taxa are placed in leaf nodes.
In a stemma, a text version that corresponds to an internal node is called a hyparchetype.
Illustration
Fig. 1. Example of a graph depicting the names of important parts of a graph or tree.
In other languages
GE: innerer Knoten (innere Ecke)
FR: sommet interne
IT: nodo interno / vertice interno