Simple Definition:

Discipline is the practice of training oneself or others to follow rules and guidelines [instructions or directions]. It involves self-control, obedience, and consistency. For example, a student who completes their homework every day without being reminded by their parents is showing discipline. Another example is an athlete who follows a strict diet and exercise routine to improve their performance.

Very Simple Definition:

Discipline is the act of following rules and being consistent. It means doing what you are supposed to do. For example, doing homework without being reminded.