Discipline has multiple meanings in the non-project related world. One meaning, regarding the act of providing firm guidance with potential punishment for failure in the context of parenting, teaching, or training, is not applicable to the world of project management. The other meaning, which would be more applicable to the world of project management, could be defined as a specific area or focus or concentration on which a specific work element is directed. In terms of project management, the term discipline refers specifically to a field of work or an area of focus in which a specific bank of knowledge is required, and in addition, a specific field of work that will require unique and unalterable sets of rules and regulations regarding the required conduct that must be adhered to in the process of completing that work activity. Some examples of specific fields of work which can be considered disciplines are computer programming, different mechanical functions, financial functions, etc.
This term is defined in the 3rd edition of the PMBOK but not in the 4th.