Let h be the height of the tree.
For a complete binary tree, following conditions holds true
a) tree is a complete binary tree till level h-1.
b) nodes at level h are continous towards the left. If you do a level order traversal last level do not contain any null node in between the nodes
For example following is almost complete binary tree
/\ / \
4 5 6 NULL (note there are no null nodes between 4,5,6)
Below is not an almost complete binary tree
4 NULL 6 NULL ( there is a null node between 4 and 6)