I'm scouring the internet for a definition of the term "Internal Node." 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.
Here are the two places I've been mainly looking:
http://planetmath.org/encyclopedia/ExternalNode.html 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.
http://www.math.bas.bg/~nkirov/2008/NETB201/slides/ch06/ch06-2.html seems to insinuate that internal nodes only exist in proper binary trees and doesn't yield much useful information about them.
What actually is an internal node!?