A node in a graph is a leaf (or a leaf node) if it has degree one and the only edge associated to the node is either undirected or is directed to it. Hence, in an undirected graph, all degree one nodes are leafs. In a directed graph, each degree one node is either a root or a leaf. It is usually required that a tree or a graph only have at most one root node.
Illustration
Fig. 1. Example of a graph depicting the names of important parts of a graph or tree.
In other languages
DE: Blatt
FR: feuille
IT: foglia