In the abundance of information, both machines and human researchers need tools to navigate and process it. Structuring and formalization of data into hierarchies, such as trees, may establish the relations between the data required for efficient machine processing and may make the information more readable for data analysts. Yet, in more complex domains, such as in natural language processing, relations between concepts go beyond simple hierarchies and form thesaurus-like networks. For such cases, researchers use ontologies as common vocabularies for specialists who need to share information in a domain. Ontologies were first defined as "explicit formal specifications of the terms in the domain and relations among them" (Gruber 1993) and, more specifically, "a formal, explicit specification of a shared conceptualization" (Studer et al. 1998) and are used in a number of applications, including the following, as specified by Noy and McGuinness (Noy and McGuinness 2001): Ontologies are the tools to provide comprehensive description of the domain of interest with respect to the users' needs It is something that we see when, for example, medical information is published on, several different websites.