title: What is an "internal node" in a binary search tree?
body: <p>I'm scouring the internet for a definition of the term &quot;Internal Node.&quot; I cannot find a succinct definition. Every source I'm looking at uses the term without defining it, and the usage doesn't yield a proper definition of what an internal node actually is.</p>
  <p>Here are the two places I've been mainly looking:
  <a href="https://planetmath.org/ExternalNode" rel="nofollow noreferrer">Link</a> assumes that internal nodes are nodes that have two subtrees that aren't null, but doesn't say what nodes in the original tree are internal vs. external. <br /></p>
  <p><a href="http://www.math.bas.bg/%7Enkirov/2008/NETB201/slides/ch06/ch06-2.html" rel="nofollow noreferrer">http://www.math.bas.bg/~nkirov/2008/NETB201/slides/ch06/ch06-2.html</a> seems to insinuate that internal nodes only exist in proper binary trees and doesn't yield much useful information about them.</p>
  <p>What actually <em>is</em> an internal node!?</p>