Good answers, but I find a defining characteristic is how they move and generate power.
Taiji is an internal martial art, but I have seen many people practice it externally - basically they don’t move right. Too much tension, too much local muscle, not really using the center, disconnections in the body so it is not moving as a unit, etc, etc.
I was once told of this guy giving a lecture on internal strength concepts that while at the blackboard and writing, he turned around to find much of the class looking at him very strangely. He asked what was up, and they said they could see his whole body was even behind writing on the chalkboard - it was not just his arm moving, as most do.
This is a defining characteristic of what is internal. How you move. Externalists move differently than internalists. Most external martial arts styles incorporate internal ideas at higher levels - I remember my karate teacher telling me things he just discovered that I had learned in tai chi. Eventhough an externalists may become more internal, most of the time it is still a far cry from what an internalist does.
What it means to be internal is a packaged set of various characteristics/ideas/techniques - but the “are you moving right” portion is usually skipped; particularly by tai chi people. If you are not moving right - you do not have an internal martial art, it is dancing, perhaps a chi gung for health, etc. but not an internal martial art.