One of the most crucial questions for service management is of course: "What is a service?".
The inventory of 100 ISO and IEC definitions of 'service' already revealed that standards have made rather a mess of this. The popular best practice frameworks do the same. They define a service in an utterly vague way like "A means of enabling value co-creation by facilitating outcomes that customers want to achieve, without the customer having to manage specific costs and risks" [ITIL 4] or "Way to provide value to customers through bringing about results that they want to achieve" [FitSM], or they limit this to only IT as in "The day-to-day provision to customers of IT infrastructure and applications and support for their use" [COBIT]. These definitions are not operable and/or not applicable at an enterprise-wide level.
USM solves this strategically with the following definition:
A service is a supported facility. The facility is specified in terms of goods and actions, and the service is evaluated in terms of functionality and functioning.
This is a universal definition, applicable to any service in any line of business.
Read the details on the service definition, its characteristics, and its decomposition into operable components in the USM Wiki.