Define the following:
(i) Tree ( recursive defination)
(ii) Level of a node.
(iii) Height of a tree.
(iv) Complete Binary tree.
(v) Internal Path length. 
Ans:

(i) Tree (recursive definition)
A tree is a finite set of one or more nodes such that.
(1) There is a specially designated node called the root.
(2) The remaining nodes are partitioned into n>=0 disjoint sets
T1, T2 ... Tn where each of these sets is a tree. T1, T2 ... Tn
are called the subtree of the root.
(ii) Level of a node
The root is at level 0(zero) and the level of any node is 1 more than the level of
its parent.
(iii) Height of a tree
The length of the longest path from root to any node is known as the height of
the tree.
(iv) Complete Binary tree
A complete binary tree can be defined as a binary tree whose non leaf nodes have
nonempty left and right sub tree and all leaves are at the same level.
(v) Internal Path length
It is defined as the number of node traversed while moving through one
particular node to any other node in the tree.