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Ontology, in Web Semantics, can be understood as an explicit definition of a context, where contextualization is a simplified view of the represented world, a collection of objects of the domain, concepts, and their relations (Jacob, 2003). Ontologies are used in artificial intelligence, information sharing, communication, interoperability, and for the reuse of knowledge domains.
In the context of Web Semantics, ontology is composed of classes, properties, relations, axioms, and instances. Classes represent the concepts of the domain. Properties correspond to the characteristics whose values differentiate instances of the same class. Relations represent the interaction between concepts or classes. Axioms define always true statements in the domain. Instances are the representations of specific elements of the concepts, which is the actual information (Ehrig & Sure, 2004).
In an algebraic definition, an ontology is a pair O=(S, A), where S is the vocabulary (classes, attributes, relations) and A are the axioms (Kalfoglou & Schorlemmer, 2003).
A major problem in the use of ontologies is the variety of existing ontologies. Many of them represent similar domains or intersections between domains, but they are modeled in different ways. This heterogeneity complicates any process that uses more than one ontology, such as software interoperability and semantic search.
Ontology mappings are constructed to deal with this problem. These mappings are rules that associate the concepts of one ontology with another. The mapping process can be defined as: “given two ontologies A and B, to map an ontology to another means that for any concept in ontology A, try to find a corresponding concept in the ontology B, with the same or similar semantic, and vice versa” (Ehrig & Sure, 2004).