What is an "internal node" in a binary search tree?

Solution 1:

     I         ROOT (root is also an INTERNAL NODE, unless it is leaf)
   /   \
  I     I      INTERNAL NODES
 /     / \
o     o   o    EXTERNAL NODES (or leaves)

As the wonderful picture shows, internal nodes are nodes located between the root of the tree and the leaves. Note that the root is also an internal node except in the case it's the only node of the tree.

What is said in one of the sites about an internal node having to have two children is for the tree to be a complete binary tree, not for the node to be internal.

Solution 2:

As far as i understand it, it is a node which is not a leaf.