In wikipedia terms you can say that
A service-level agreement is a negotiated agreement between two parties, where one is the customer and the other is the service provider. This can be a legally binding formal or an informal "contract" (for example, internal department relationships). Contracts between the service provider and other third parties are often (incorrectly) called SLAs – because the level of service has been set by the (principal) customer, there can be no "agreement" between third parties; these agreements are simply a "contract." Operational-level agreements or OLAs, however, may be used by internal groups to support SLAs.
