Not the best in translation from Simplified Chinese to Pinyin to English, (but at least refers to the physical) yields:
内功 [nèi ɡōnɡ] Pinyin
"exercises to benefit the internal organs, internal energy or power, internal capabilities"
I am never happier as a teacher than when I hear people both within and outside the Tai Chi community ask what "Internal Discipline" is. Although it is still perplexing that Tai Chi afficionados do not know what it is. After all Tai Chi has been known for a long time as an "Internal" martial art. A typical question from even long time Tai Chi practitioners is "what is internal discipline".
Perhaps a literal definition would be appropriate to start things off. I do not like what Merriam-Webster had to say. I find that the "Free Dictionary" has this among several other definitions which I do like and find appropriate to Tai Chi: Discipline is : "training or conditions imposed for the improvement of physical powers, self-control, etc".
Note the use of the word "physical" which is particularly encouraging because most definitions only dwell on "mental". This, sadly to say is also the proclivity of most Tai Chi practitioners that I have heard over the years. I hear over and over and in many formats that Tai Chi not only cultivates but depends on a particular "mental" state. For the most part this is roughly defined as being one of relaxation, serenity, placidity, free of tension, "be cool", etc. Most, then expand this further to (as my teacher says) "an unspoken belief that as long as one has the right mental state, almost any Tai Chi movement is a Tai Chi movement" p. ii, "Uncovering the Treasure". This, he seems to feel is the direct cause for "infinite varieties of Tai Chi that have sprouted everywhere". Having heard and seen what he is speaking about I must say the use of the word "sprouted" is certainly not lost on this writer.
In light of the phenomenon that I describe I find that in going a little further I not only have to define "internal discipline" to people but I have to show it/demonstrate it. For in the explanations, I have found no justice. I inevitably end up showing it to both beginners and afficionados as I try to explain what I am doing. What is even more surprising to me is that even though "internal discipline" translates to the word or term called "Neigong". Long time practitioners do not understand the physical. That term has been in the Tai Chi lexicon for quite some time yet even the online martial art encyclopedia definitions still lapse into such things as: "Neigong, also spelled Nei Kung, neigung, or nae gong, is any of a set of breathing and spiritual practice disciplines associated with Taoist religion and Chinese martial arts".
I have a feeling my teacher realized the difficulties associated with explaining what it is and instead opted to explain HOW IT DOES: (From the Classical Tai Chi website)"The central element of the practice dictates how a movement should be made from the internal core of the body—the abdomen and the back—not from the external parts of the body, such as arms and shoulders".
AND WHAT IT DOES: "Internal Discipline enables you to initiate movements from the internal core of the body (the abdomen and back) rather than from the external parts of the body (the limbs), and cultivates and mobilizes your internal energy for health benefits and martial arts applications".