Interoperability is a very imprecise and often ill-defined concept. Regularly, definitions reference the IEEE glossary which defines interoperability to be “the ability of two or more systems or components to exchange information and to use the information that has been exchanged” . Interoperability thus is characterised as an actively exerted skill of software systems or sub-systems (i.e., tools, modules, and the like). Furthermore, emphasis is being laid on mutual communication of information and the ability to apply this communicated information. Considering contemporary understandings of “in-formation’, the latter part is redundant, as information already is defined to be data put into use .
Christian  defines that interoperability is “a condition that exists when the distinctions between information systems are not a barrier to accomplishing a task that spans multiple systems.” Leaving open, whether its software systems or humans interacting with machines, this definition stresses that it’s a higher level goal that drives interoperability (“task’). Potential “distinctions between information systems’ emphasise that these systems can be similar (e.g., same tool in different places or organisations) or heterogeneous (e.g., facilitating different activities). Interoperability is defined as the necessary minimum cooperation between these systems allowing for fulfilment of a task which can only be carried out by combining these systems.
The capability to be interoperable is often mixed up with a couple of related concepts, encompassing “coexisting’, “interconnectable’, “interworkable’, and “interchangeable’ (see Figure 1).
Figure 1. Dissocation staircase (modified from Kosanke, ).
By assessing capabilities along the three layers communication & control, (meta-) data, and business model, interoperability can easily be differentiated from the other notions. If none of the features is provided by two communicating systems, they are obviously incompatible. By using the same communication protocols, systems can be classified as coexistent. If they additionally offer the same communication interface and allow for (meta-) data access, applications can be called interconnectable. By furthermore providing identical (meta-) data types, they are interworkable. Interoperability adds cooperation on the application semantics and application functionality level. If they also support the same business model, processes, policies, etc. they can be called interchangeable.
To sum up, interoperability can be defined as follows. Interoperability is a property that emerges, when distinctive information systems (subsystems) cooperatively exchange data in such a way that they facilitate the successful accomplishment of an overarching task.
– Stefan Sobernig, Fridolin Wild
 Karpatschof, B.; Human Activity, Contributions to the Anthropological Sciences from a Perspective of Activity Theory, Dansk Psykologisk Forlag, Copenhagen, 2000.
 Christian, E.; The Government Information Locator Service (GILS): Report to the Information Infrastructure Task Force. IITF Committee on Information Policy; 1994.
 Kosanke, K.; ISO Standards for Interoperability: a comparison. In Pre-Proceedings of the INTEROP-ESA 2005, pp. 59–67, Geneva, 2005.