II. BACKGROUND KNOWLEDGE
A. Semantic
The definition of semantic is meaning. The semantic of something is a meaning of something. Semantic is used to solve the problem of words ambiguity: There might be more than a meaning of the word. Therefore, semantic technology is used to identify the exact meaning of the word or item requested. Semantic search is a process used to improve searching by using data from semantic networks to disambiguate queries and web text in order to generate more relevant results.
B. Ontology
Ontologies are widely used in knowledge engineering, artificial intelligence; as well as applications related to knowledge management, information retrieval and the semantic web. Among the first definition of ontology was provided by Neches et al. [2]; which said that "an ontology defines the basic terms and relations comprising the vocabulary of a topic areas as well as the rules for combining terms and relations to define extensions of the vocabulary". However, the most quoted definition of ontology is based from Gruber [3]; which defined it as "an explicit specification of a conceptualization". Although, there has been no universal consensus for the definition of ontology; the aim of ontology is very clear as put forward by Gomez-Perez et al. [4] that is "to capture consensual knowledge in a generic way, and that they may be reused and shared across software applications and by groups of people". What considered as an ontology is still a debatable issue. Lassila&McGuinness [5] provides a broad categorization ontology, of which can be categorized as lightweight and heavyweight ontology. The ACM topic hierarchy used in this study can be referred to as a lightweight ontology
c. Digital Library
In the computer science research community, the term of digital library is viewed as content collected on behalf of the user. According to Gladney H.M et. Al [6], a digital library service is an assemblage of digital computing, storage and communications machinery together with the software needed to reproduce, emulate and extend the services provided conventional libraries based on paper and other materials means of collecting, storing, cataloguing, finding and disseminating information.