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Description Logics are a set of formal languages, which can model concepts, roles, relations and individuals, used for formal knowledge representation. They are structured and decidable fragments of first order logic and are more expressive than propositional logic. They are a family of knowledge representation formalisms that are used in describing domains through the concepts contained in them in and their objects, object properties and instances. (Baader, F., 2003, p. 47)

Description Logics provide a logical formalism to knowledge representation languages such as UML and others. This is particularly useful for ontologies because the formal representation of knowledge provided by Description Logics can be effectively accessed and reasoned over. Additionally, the limited vocabulary of Description Logics compared to first order logics improve the efficiency of automated reasoners and ensure decidability. They differ from first order logic by being function-free; no variables are present in the formalism and at most three at the predicate level.

Due to the impact of Description Logics on ontologies as well as terminologies, formal conceptual data modelling and information integration it has applications in, most notably, Artificial Intelligence and the Semantic Web.

Reasoning using Description Logics
Reasoning (inferring implicit information in an ontology from its defined concepts and object properties) is an important feature in formalised Description Logics over First Order Logics which may not be decidable. (Baader, F., 2003, p. 47) Automated reasoning, made possible by the decidable fragments of First Order Logics that constitute Description Logics, automates the ability to make these inferences which makes reasoning (a primary feature of logics) scaleable. The unambiguous semantics of Description Logics are necessary for reasoning services to be employed. (Keet, C. M., 2015)

Often there is a trade-off between expressivity of the description logic and the complexity of the reasoning algorithm or the ease of which the logic can be reasoned over. With reasoning as an important goal of description logics, many different description logics exist to balance the complexity and performance of the algorithm being used with what can be expressed with the logic.