Summary: Coordination models like LINDA were first conceived in the context of closed systems, like high-performance parallel applications. There, all coordinated entities were known once and for all at design time, and coordination media were conceptually part of the coordinated application. Correspondingly, traditional formalisations of coordination models -- where both coordinated entities and coordination media are uniformly represented as terms of a process algebra -- endorse the viewpoint of coordination as a language for building concurrent systems. The complexity of today application scenarios calls for a new approach to the formalisation of coordination models. Open systems, typically hosting a multiplicity of applications working concurrently, require coordination to be imposed through powerful abstractions that (i) persist through the whole engineering process -- from design to execution time -- and (ii) provide coordination services to applications by a shared infrastructure in the form of coordination media. As a unifying framework for a number of existing works on the semantics of coordination media, in this paper we present a basic ontology and a formal framework endorsing the viewpoint of coordination as a service. By this framework, coordination media are characterised in terms of their interactive behaviour, and are seen as primary abstractions amenable of formal investigation, promoting their exploitation at every step of the engineering process.