Most were developed for a specific discipline or a specific product or family of products. With the advent of natural language processing, there have been significant developments in semantic networks. These KOSs structure concepts and terms not as hierarchies but as a network or a web. Concepts are thought of as nodes, and relationships branch out from them. The most noted semantic network is Princeton University's WordNet, which is now used in a variety of search engines.
Ontology is the newest label to be attached to some knowledge organization systems.
The knowledge-management community is developing ontologies as specific concept models. They can represent complex relationships among objects, and include the rules and axioms missing from semantic networks. Ontologies that describe knowledge in a specific area are often connected with systems for data mining and knowledge management . All of these examples of knowledge organization systems, which vary in complexity, structure, and function, can provide organization and increased access to digital libraries. To provide multiple access points beyond the limits of a single physical location, subject headings are applied.