For a long time, it was highlighted that the service is intangible or immaterial; these characteristics are mainly used to distinguish the service, making it incomparable with good. The set of proposed definitions and characteristics that led to debates between specialists in economics for several years and gave place to a variety of visions and approaches are still not consensual and science engineers challenge. Goal of this paper is to present, on the one hand, the existing literature proposed by economists concerning services (definitions and specificities) and, on the other, arguments proposed by science engineers that challenge them. That study allows us to propose a generic definition of what a service is and a preliminary model of a service activity.