What is "internal" martial art?
I can't resist saying "I don't know, go ask an expert".
Let us see if my understanding thus far is helpful to someone else.
The internal aspect is something that can't be seen but can be felt. Much like the Force. I have yet to experience the Force, but in Wing Tsun sensing movement that can't be seen is often present during ChiSao. As an example: if I place my arm on yours and tense my muscles enough to apply a slight force, others watching would not have anything external to see, but it is something you and I could feel - internal. I like that example, but it is not a full explanation.
I was lucky to have a Yiquan instructor this summer. Yiquan is an internal martial art that I hope to keep training. I like that its focus is on training the mind. Since that is vague, here is am example I like: similar to just before shooting a free throw you should imagine making it. Sprinters imagine running fast, field goal kickers making it, etc. Yiquan takes this idea of imagery and applies it in martial arts. It has much more than that of course, but I easily add that to my WT.