Discipline is doing what needs to be done, even when it isn’t convenient or comfortable. To be disciplined, a person must gain control over their emotions. Emotions are necessary for envisioning a goal and finding passion, but they tend to get in the way of the day to day things that need to happen to achieve a goal.
For example, an undisciplined person who has set a goal to lose ten pounds will decide whether or not to work out based on their current emotion. A disciplined person may feel sad or tired also, but they still choose to work out despite their emotional state.