1. Binary Tree : A binary tree is either empty, or it consists of a node called the root together with two binary trees called the left subtree and the right subtree of the root.

# Predecessor : Every node in a binary tree, except the root, has a unique parent, called the predecessor of the node.

# Descendent : Children is called a descendent of node Parent.

# Ancestor : Parent is called an ancestor of child.

# Edge : Line through two nodes is called an edge.

# Path : Sequence of consecutive edges is called a path.

# Complete Binary Tree : is a binary tree that have at most two childrens.

# Extended Binary Tree : is a binary tree if each node has either 0 or 2 children.

(a). Internal Nodes : In a extended binary tree, the nodes with two children are called internal nodes.

(b). External Nodes : nodes with 0 children.