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, (N1,...,Nk) such that Nk is also connected to N1 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.
Fig. 1. Example of a graph depicting the names of important parts of a graph or tree.