Service-level agreements (SLAs) are contracts between service providers and customers that define the services provided, the metrics associated with these services, acceptable and unacceptable service levels, liabilities on the part of the service provider and the customer, and actions to be taken in specific circumstances.
Source: Dobler, D.W., & Burt, D.N. (1996). Purchasing and Supply Management. (6th ed.). New York: McGraw Hill.