The problem is that it changes depending on whom you ask. Some say it's the three arts called "internal" by author and martial artist, Sun Lutang--Taiji, Xingyi and Bagua. Some say it's a focus on minute body mechanics--I say this is no good because then most good martial arts qualify, rendering the term meaningless. Others will say that anything involving softness and redirection should be considered "internal," citing Taiji and Aikido. I disagree, because most grappling arts have these concept and what, exactly is "internal" about them? My personal definition is "an art that combines cultivation of the energy (qi) and other such exercises based on an 'energy theory' of the body--like that used in acupuncture." Most traditional CMAs practiced in a traditional way will qualify, most non CMAs do not, simply because they don't have qigong/energy work. Perhaps some Indian arts which combine Yoga-like techniques with their art might qualify, but I don't know enough about them to comment. That's just my definition. Like I said, you won't get an even remotely unified answer.
There are a couple ways of translating "internal." Internal can mean using power generated from correct position and form, instead of more obvious physical power generated from muscle mass. It can also mean that the practice is "soft," full of slow movements, meditation, and very little calisthenics to build muscle, and little to no working on sparring with partners who kick, punch, or grapple. Tai-chi is probably the most common example of this. Another way has to do with the impression the Chinese had of martial arts. Internal meant styles developed inside China, and covered many different ways of training and fighting, and external meant styles developed outside of China.
Traditional CMAists do not actually make a big deal of this distinction. The term is an overly-literal translation of the Chinese term "Neijia-quan," literally, "inside-the-house boxing." This was in reference to the fact that this style was only taught to the bodyguards of the imperial household, hence, only for people "inside the house." This martial arts is now extinct, though some think it may have been a predecessor to modern Tongbei-quan. The next time this term seems to pop up is like 400 years later when Sun Lutang calls the arts he practices, Taiji, Xingyi and Bagua, the "Neijia-quan," collectively because he felt they shared certain similar principles. It's really not a very useful term since most accurately it should refer to a now-extinct martial art and because Taiji, Xingyi and Bagua, while they do share some characteristics, are also not unique in the CMA world in their emphasis on softness, energy cultivation, etc. I really wish people would stop using the term altoghether and just describe the specific art they're doing.
IMO... "Inside the House" also means within the being ie the person whereas "waijia" refers to the body and its outside characteristics and to the outside effects of MA - to me internal really means emphasising development deeper than just muscles and physics (but including all that) and not just simple fighting techniques (but including all that...) I think using the term to refer to the Tufty Clubs of CMA is a modern take on an old idea tbh and comes from the PRC versions of history that always want to avoid any kind of religion, philosphy or mysticism for political reasons... to sum up: Internal MA = deeper gung fu (imho) :Angel:
I am with onyomi on this in terms of disliking the term. I no longer say I do an internal martial art, but Tai Chi. I think the term only distracts. Perhaps I have just got bored with the arguments, but I have no will to argue the point with anybody. I'm just doing what I am doing.
Or even: "I am what I do" or "I am doing what I do by not doing" - those are good "internal" terms imo...
I yam what I am an' dat's all dat I yam,I'm Popeye the sailor man. Yeah,it's a catagorization that doesn't make sense in regards to CMAs 'cause there's too much overlap of theory,technique,training methods and smaller physical mechanics. And it's never going away. But dear friends,let us take delight in this fact. This forum is here due to that term.
"I yam what I am an' dat's all dat I yam." I go with that... And Popeye could KICK A** too All the best. Robert.