For a pithy quote, I once read discipline defined as, "Eliminating the bad to make room for the good, and eliminating the good to make room for the great."
I think the conflation with "punishment" has made it very difficult to teach self-discipline to kids and young adults.
I learned “discipline” from the martial art world. Someone had pointed out to me that “discipline” shares the root word of “disciple”, so it is not really about punishment. With that meaning in mind, you don’t discipline a child so much as initiate a child into a discipline.
Related things I learned from the martial art world are, “don’t cheat yourself”, and the idea of gongfu — mastery accumulated over time. And to bring some perspective to this, I also learned from the martial art world that “play” is crucial to develop the creative aspect of mastery. Some teachers will tell students to “play” with an idea, instead of running through a drill.
I think the conflation with "punishment" has made it very difficult to teach self-discipline to kids and young adults.