“discipline, n. II. Senses relating to training, instruction, or method. 5b. A particular school or method of instruction; an educational philosophy.  6. A system or method for the maintenance of order; a body of rules for conduct or action; a way of doing things. 7a. A branch of learning or knowledge; a field of study or expertise; a subject. Now also: a subcategory or element of a particular subject or field. b. spec. A branch or field of sporting activity; a subcategory or element of a particular sport.” OED

What this means:

When used in academia, discipline refers to the field of study. Mathematics is an example of a discipline, while statistics, calculus, and algebra are subjects within the mathematics discipline. Other examples of disciplines are life sciences (subjects: biology, chemistry, botany, etc.) and social sciences (subjects: psychology, sociology, etc.).