A service in Service Oriented Architecture can be defined in various ways. Given below are the common definitions:
Basically, services are a group of methods that share a common set of requirements and functional goals. They are called by other parts that need to execute its logic, depending on the outcome (such as data, results of calculations, and so on). The functions have a clearly defined public signature which is published so other code (service clients) can use the functions in the service as a black box. The service operations are invisible — there is no direct interaction with a user and the work is executed as instructed by the given input parameters.
Services are core business logic that are protocol-independent, location-agnostic and contain no user state. They are coarse-grained (it can perform its logic and return the result in a single call). Services may be reused across diverse applications.
A Service is a process or technique by which a consumer's requirement is satisfied according to a negotiated contract which includes Service Agreement, Functions Offered and so on.
A Service is the seeking and receipt of a specific outcome of a customer across a range of interactions and touch-points over time.
A Service is basically a process with an interface that permits asynchronous communication with other services.