Description Logics (DLs) are knowledge representation languages built on the basis of classical logic. DLs allow the creation of knowledge bases and provide ways to reason on the contents of these bases. Fuzzy Description Logics (FDLs) are natural extensions of DLs for dealing with vague concepts, commonly present in real applications. Hájek proposed to deal with FDLs taking as basis t-norm based fuzzy logics with the aim of enriching the expressive possibilities in FDLs and to capitalize on recent developments in the field of Mathematical Fuzzy Logic. From this perspective we define a family of description languages, denoted by ALC*(S), which includes truth constants for representing truth degrees. Having truth constants in the language allows us to define the axioms of the knowledge bases as sentences of a predicate language in much the same way as in classical DLs. On the other hand, taking advantage of the expressive power provided by these truth constants, we define a graded notion of satisfiability, validity and subsumption of DL concepts as the satisfiability, validity and subsumption of evaluated formulas. In the last section we summarize some results concerning fuzzy logics associated with these new description languages, we analyze aspects relative to general and canonical semantics, and we prove some results relative to canonical standard completeness for some FDLs considered in the paper. © 2010 Elsevier Inc. All rights reserved.

he authors acknowledge support of the Spanish projects MULOG2 TIN 2007-68005-C04-01/04, ‘‘Agreement Technologies” (CONSOLIDER CSD 2007-0022, INGENIO 2010) and the grants 200750I005 from CSIC, and 2009-SGR-1433/-1434 from the Generalitat de Catalunya. The authors also want to thank Félix Bou and Pilar Dellunde for their helpful comments and suggestions.

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