These people can control their thoughts, emotions, and actions. They’re hard-working, energetic, and able to motivate themselves to strive towards the best outcomes in many aspects of their lives.
Like Careful people, they can easily resist temptation and control their urges. While Carefulness is defined more by impulse control and having a measured response to things, discipline is defined by an ability to get stuff done without getting sidetracked by distractions or obstacles.
These people feel that practice is as important as performance, and can always say "enough is enough." They're always prepared and because they have a unique ability to ignore distractions or boredom, they can motivate themselves to complete tasks regardless of whether they want to or not.
"Discipline is doing what you hate to do, but nonetheless doing it like you love it." - Mike Tyson.