A tree is a graph with exactly one path between any two vertices, so it contains no cycles. Removing an edge from a cycle can turn a graph into a tree. section 4.3
Figure 4.6. In (a), a cycle exists between vertices B, C, and D, so it is not a tree. In (b), the edge connecting C and D has been pruned, so the graph is a tree.
The diagram contrasts a cycle through B, C, and D with the tree formed by removing the edge between C and D. section 4.3










