Co-tree

Co-tree
The branches of a graph which are not included in a tree form a co-tree. A co-tree is a complement of a tree.

Chords or Links
The branches of a co-tree are called links or chords. 

Tree

Tree
A tree of a connected graph is defined as a set of branches which are connected , contains no circuits, contains all nodes of the graph. In simple a tree connects all the nodes of the graph without forming a closed path.

Twigs
Branches of tree are called twigs.

Path

Path
It is a subgraph of a given graph, such that we can proceed from one node to the other by traversing the branches of the subgraph in a sequential manner.

Incident Branch

Incident Branch
A branch k is called incident to a node l if the node l is an end point of the branch k.

Subgraph

Subgraph
A subgraph of a graph is a subset of the branches of the graph.

Node

Node
The end point of a branch is called is a node. The number of nodes in a graph is denoted by n.

Branch

Branch
Each oriented line segment of the graph is called branch. The number of branches in a graph is denoted by b.