Domain is the perspective and the territory of the discipline. It contains the subject matters of a discipline, the main agreed upon values and beliefs, the central concepts, the phenomena of interest, the central problems, and the methods that are used to provide some answers to questions in the discipline. Perhaps think of a domain as an area of study that is identified by a common phenomenal boundary (Kim, 1998, 2000).

It is widely considered that the domain of nursing has as sub-domains, client or patient, client-nurse interaction, practice and environment. Within those sub-domains one finds the phenomena of nursing, person, health and environment.

Kim,  H. S. (1998). Reflections on "Structuring the nursing knowledge system: A typology of four domains." Scholarly Inquiry for Nursing Practice, 12(4), 383-388. 
Kim,  H. S. (2000). The nature of theoretical thinking in nursing (2nd ed.). New York, NY: Springer Publishing Company.