Generally speaking, discipline is the ability to obey consistently to a system, code or philosophy, and apply its teachings. Most of the time, this comes with a price, which is the rejection and suppression of temptations and desires. This is why discipline, while highly important, can also be very difficult for those who lack restraint and tenacity.
The art of discipline expresses itself in various ways, whether by obeying current authority or pursuing a goal in solitude. One can say that discipline comes into practice when a child becomes a student in school. They have to obey their teachers, do their homework, and follow the rules of the institution.
The more the child obeys and does their obligations, the more disciplined they become. Thus, on the interpersonal level, discipline serves a basic function in a social order or construct: The function of complying to instructions, rules and the application of their lessons. The more disciplined the members of a construct are, the more productive and efficient the construct is. By being more disciplined to something external to you personally, you cancel your own urges in its favor. This allows you to survive and potentially prosper in an external system.
Should you be discilpined enough in a social order as its subordinate, you have a chance to be recognized for your efforts, and even climb up its ranks. Thus, being a good disciple of something other than yourself, can reward you, giving you interest to stay disciplined.
As already mentioned, discipline also serves a role in solitude, and because it is not necessarily dependent on other people, it is called self-discipline. Self-discipline is the ability to obey oneself persistently at the price of avoiding pleasure and laziness. While the external disciple is loyal to something and/or someone else, the self-disciple is loyal to either themselves or to their own beliefs.
As such, discipline is expessed by act, and does not exist as being. Thus, the issue of discipline is one that is based on respect, as respect is measured by practice, and not by love, which aspires be one with a being. No, the disciple is one that has to prove their discipline by their actions, as their acts serve as proof to their compliance of a code/system/someone else.
As an individual who very much appreciates self-discipline, I believe it serves as a better motivation than simply a certain urge, and it is very important to maintain, specifically in solitude, because in solitude no one might be there to take care of you. The disciplined man or woman may act not on instinct on desire but on a rationalized motive. For example, we may work regardless of our desires, because the rationality behind our actions is to survive and earn money.
Thus, the self-disciplined person ought to take care of themselves, whether it is the maintenance of the body, pursuing a hobby, or doing a project. This can be regardless of what they feel at the moment. When your rationality overcomes your short-term temptations, that is an expression of discipline, if it's applied practically.
One can say that discipline is the antithesis opposite of laziness, which I believe to be true. That is because laziness is expressed by the submission to urges that tempt you to not act on the ability to apply reason. The lazy individual is prone to damage their health and not fulfilling their reason, by pursuing passions that do not promote their commitments. Playing video games for long periods of time, instead of working, while being supposed to work, is such an example.
The lazy person is not disciplined because they lack the will or tenacity to exert effort to do something which is not fun or grant them pleasure. I tend to believe that children are usually very lazy, mostly today. Instead of indulging in physical exercise, they usually spend their time in front of a computer or a TV screen, and thus some of them get obese and weak. Laziness can be compared to a demon which seduces you to harm yourself and your body by submitting to pleasures instead of effort. It's even true in religious context.
One of the most advantageous things about discipline is the ability to liberate oneself from one's current desires and overcome them in the name of a higher value or goal, thus making the disciplined person a master of themselves in the name of rationality. Through denial to act on impulse, we are able to conquer it, and thus, ourselves. More specifically -- our id-ego.
To be disciplined is to know and to exercise the notion that there are things more important in life than simply hanging around or slacking. Things more important, even, from our temporary emotions. This is why self-discipline is also important when leading a healthy lifestyle of good nutrition and physical exercise. The healthy person overcomes the temptation for candies, cigarettes, alcohol, and so forth, and the athlete overcomes their will to skip a training session.
Discipline is also very vital when growing up mentally, and therefore high levels of it can indicate a mature personality. The mature individual knows that they have obligations, whether to themselves or to other people, and they are the ones to take responsibility for their actions. The disciplined person also knows this, and therefore they are mature individuals.
An example of self-discipline can be seen in my own life when I used to be more-physically active. I made an obligation to myself to go to the local gym once or twice a week, regardless of whether I wanted to or not, or if I preferred to do something else at a certain moment. Except for one time when I skipped the gym due to muscle pain, I kept my obligation and plan to do so until the end of my membership.
I know that going to the gym is important because it is good for my health and physical maintenance. Thus, being disciplined means doing your deeds not because you necessarily want to do them, but because you are aware of their functionality, whether to yourself or to society.
When you act on the functionality of the teachings you follow, AKA, the codes which you submit yourself to, voluntarily or not, it is then when you know you are capable of doing more than what you otherwise want to to do. We need to understand that our wants do not always matter in this world, which can be harsh and merciless.
And we need to do whatever we can to survive, as survival allows us to prosper. And in order to survive, we must be fitting enough to our lives, and that is done by being disciplined. Canceling ourselves in this world may be more important than some of you might think.