In a graph, two nodes are said to be *adjacent* if they are connected by an edge. A *cycle* is a sequence of adjacent nodes, i.e., a path, (*N _{1},...,N_{k}*) such that

*N*is also connected to

_{k}*N*by an edge. In a directed graph, if all the edges satisfying the above definition are traversed in the forward direction, the cycle is said to be directed. There are by definition no cycles in trees. In textual criticism cycles turn up in the case of contamination.

_{1}#### Illustration

Fig. 1. Example of a graph depicting the names of important parts of a graph or tree.