Citation Network is a social network which contains paper sources and linked by co-citation relationships. Egghe & Rousseau once (1990, p. 228) explain "when a document di cites a document dj, we can show this by an arrow going from the node representing di to the document representing dj. In this way the documents from a collection D form a directed graph, which is called a 'citation graph' or 'citation network' ".
Citation is a reference to a published or unpublished source (not always the original source). More precisely, a citation is an abbreviated alphanumeric expression embedded in the body of an intellectual work that denotes an entry in the bibliographic references section of the work for the purpose of acknowledging the relevance of the works of others to the topic of discussion at the spot where the citation appears. Generally the combination of both the in-body citation and the bibliographic entry constitutes what is commonly thought of as a citation (whereas bibliographic entries by themselves are not). References to single, machine-readable assertions in electronic scientific articles are known as nanopublications, a form of microattribution.
Citation networks, the principal focus of this study, are one kind of social networks that have been studied quantitatively almost from the moment citation databases first became available. In 1965, Derek J. de Solla Price described the inherent linking characteristic of the SCI in his seminal paper titled "Networks of Scientific Papers". The links between citing and cited papers became dynamic when the SCI began to be published online. In 1973, Henry Small published his work on co-citation analysis which became a self-organizing classification system that led to document clustering experiments and eventually what is called "Research Reviews".