the widespread use of the term "internal" as relates to TCMA was popularized via the writings of Sun Lu Tang in an apparent attempt to differentiate between what he practiced and other styles, with the elitist subtext (that seems to have carried on to this day) "what we do is better and why" (Tim Cartmell writes about this on his website in detail); otherwise, it denotes an approach to training that focuses on certain aspects of body mechanics during training than other systems do, but that's about it; oh, and all the "qi" stuff, there's some "qi" stuff in there
in fact, notice how it's almost always the "internal" guys who wax philosophic about why what they do is healthier, deadly, more authentic? how they almost always seem to go on about how the way they move involves more than "just using muscles", as if, for some reason, using muscles is a bad thing (never-mind that, regardless of what anyone thinks, if you are standing up, moving, or, hello, breathing, you are using muscles - of course, the WAY you use your muscles can be radically different from one person to the next; it also is one thing to go on about how you can throw someone without using muscular exertion when that person is inherently compliant and trained to go with what you offer them versus someone not invested in you being t3h d3adly); OTOH, the "external" guys just seem to want to fight you to make their point (barbarians, all of 'em, I say)