It is difficult for epidemiologists to agree on one single definition of the field but a frequently cited definition describes epidemiology as the "study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to control of health problems." (Last, 1995).
John M. Last, ed., A Dictionary of Epidemiology, 3d ed. (New York: Oxford University Press, 1995)
Council for State and Territorial Epidemiologists