Real Dictionary

discipline
countable noun: A discipline is a particular area of study, especially a subject of study in a college or university.
uncountable noun: Discipline is the practice of making people obey rules or standards of behaviour, and punishing them when they do not.
uncountable noun: Discipline is the quality of being able to behave and work in a controlled way which involves obeying particular rules or standards.
variable noun: If you refer to an activity or situation as a discipline, you mean that, in order to be successful in it, you need to behave in a strictly controlled way and obey particular rules or standards.
verb: If someone is disciplined for something that they have done wrong, they are punished for it.
verb: If you discipline yourself to do something, you train yourself to behave and work in a strictly controlled and regular way.

discipline in American English
a branch of knowledge or learning
to subject to discipline; train; control
noun: training to act in accordance with rules; drill
transitive verb: to train by instruction and exercise; drill

discipline in British English
to improve or attempt to improve the behaviour, orderliness, etc, of by training, conditions, or rules
noun: training or conditions imposed for the improvement of physical powers, self-control, etc