
Note:

Term  Meaning 

node  The vertices that form the hierarchical structure 
root (or "root node")  The topmost node in a tree 
edge (or link)  The connection from a level k node to a level k+1 node 
Graphically:
Term  Meaning 

Parent node  The node that is immediately above in the tree 
Child node  The node that is immediately below in the tree 
Graphically:
Term  Meaning 

Internal node  a node that has at least 1 child node 
External (leaf) node  a node that has no (zero) child node 
Graphically:
Term  Meaning 

Path from a node x to a node y  The edges that connects node x to node y 
length of a path  The number of edges in the path 
height of a tree  The longest path from the root to a leaf node in the tree 
Graphically:
Term  Meaning 

Height of a node x  The length of the longest path from the node x to a leaf node in the tree 
Depth of a node x  The length of the path from the root node to the node x 
Graphically:
Note:

Term  Meaning 

Subtree rooted at a node x  The portion of the tree consisting of the node x and all the children nodes of x and their children nodes and so on. 
Graphically: