In pervasive computing, a set of smart devices communicate and collaborate together in order to provide adapted services to the user and applications, and help the former in his everyday life tasks. To adapt provided services, devices must be context-aware, which requires a good understanding and use of context. This term is still general, vague and doesn’t help developer of context-aware applications in spite of the large number of proposed definitions. In this paper, we provide a service based definition of context that is abstract enough to be used in pervasive computing and limit the set of information required to make service adaptation. We use some scenarios to make clear our definition. We present also a new definition of context-awareness and a categorization of context based on the concept of service, which seems, to us, a fruitful approach in pervasive computing. Finally, we discuss our approach and contribution.