Modularity in ontologies
The Landscape of Modular Ontologies In the past 10 years, modularity has been established as a central research topic in ontology engineering. Several approaches and techniques have been investigated and developed in detail to characterise and deal with modularity  . Theoretical as well as practical aspects of modularity have become essential to the design of ontologies. They aim at reducing complexity, improving maintenance, and support reasoning over modules  . However, the field is
... r, the field is still highly active and widely accepted solutions are yet to be determined. The 4 articles in this special issue present thoroughly investigated approaches that contribute to modularity in ontologies on quite distinct, but equally important layers, which we will sketch below in more detail. The number of ontologies available nowadays, as well as their size, is steadily increasing. There is a large variation in subject matter, level of specification and detail, intended purpose and application. Ontologies covering several domains at once are often developed in a distributed manner addressing the various aspects, but also such that contributions from distinct sources may be relevant for different parts of a single domain. Not only is it difficult to determine and define interrelations between such distributed ontologies, it is also challenging to reconcile ontologies which might be consistent on their own but jointly inconsistent. Further challenges include extracting the relevant parts of an ontology, re-combining independently developed ontologies in order to form new ones, determining the modular structure of an ontology for comprehension, and the use of ontology modules to facilitate incremental reasoning and version control. Still catching up with 40 years of related research in software engineering (cf. e.g. [3, 4] ), modularity in ontologies is envisaged to allow mechanisms for easy and flexible reuse, generalisation, structuring, maintenance, collaboration, design patterns, and comprehension  . Applied to ontology engineering, modularity is central not only to reducing the complexity of understanding ontologies, but also to maintaining, querying and reasoning over modules. Distinctions between modules can be drawn on the basis of structural, semantic, or functional aspects, which can also be applied to compositions of ontologies or to indicate links between ontologies. In particular, reuse and sharing of information and resources across ontologies depend on purposespecific, logically versatile criteria. Such purposes include 'tight' logical integration of different ontologies (wholly or in part), 'loose' association and information exchange, the detection of overlapping parts, traversing through different ontologies, alignment of vocabularies, as well as module extraction possibly respecting privacy concerns and hiding of information, etc. Another important aspect of modularity in ontologies is the problem of evaluating the quality of single modules or of the achieved overall modularisation of an ontology. Again, such evaluations can be based on various (semantic or syntactic) criteria and employ a variety of statistical/heuristic or logical methods.