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An example of a semantic network

A semantic network is often used as a form of knowledge representation. It is a directed graph consisting of vertices, which represent concepts, and edges, which represent semantic relations between the concepts.

Semantic networks are a common type of machine-readable dictionary.

Important semantic relations:

• Meronymy (A is part of B, i.e. B has A as a part of itself)
• Holonymy (B is part of A, i.e. A has B as a part of itself)

• Hyponymy (or troponymy) (A is subordinate of B; A is kind of B)
• Hypernymy (A is superordinate of B)

• Synonymy (A denotes the same as B)
• Antonymy (A denotes the opposite of B)

An example of a semantic network is WordNet, a lexical database of English.

WordNet involves fairly loose semantic associations, compared to other more formal networks. It is possible to represent logical descriptions using semantic networks such as the Existential Graphs of Charles S. Peirce or the related Conceptual Graphs of John F. Sowa. These have expressive power equal to or exceeding standard first-order predicate logic. Unlike WordNet or other lexical or browsing networks, semantic networks using these can be used for reliable automated logical deduction. Some automated reasoners exploit the graph-theoretic features of the networks during processing.

"Semantic Nets" were first invented for computers by Richard H. Richens of the Cambridge Language Research Unit in 1956 as an "interlingua" for machine translation of natural languages. They were developed by Robert F. Simmons at System Development Corporation, Santa Monica, California in the early 1960s and later featured prominently in the work of M. Ross Quillian in 1966.

There are also elaborate types of semantic networks connected with corresponding sets of software tools used for lexical knowledge engineering, like the Semantic Network Processing System (SNePS) of Stuart C. Shapiro or the MultiNet paradigm of Hermann Helbig (MultiNet is an acronym for "Multilayered Extended Semantic Network"). The latter is especially suited for the semantic representation of natural language expressions and used in several NLP applications.

One can consider a mind map to be a very free form variant of a semantic network. By using colors and pictures the emphasis is on generating a semantic net which evokes human creativity. However, a fairly major difference between mind maps and semantic networks is that the structure of a mind map, with nodes propagating from a centre and sub-nodes propagating from nodes, is hierarchical, whereas semantic networks, where any node can be connected to any node, have a more heterarchical structure.

In the 1960s to 1980s the idea of a semantic link was developed within hypertext systems as the most basic unit, or edge, in a semantic network. These ideas were extremely influential, and there have been many attempts to add typed link semantics to HTML and XML.

• Semantic Networks - John F. Sowa: http://www.jfsowa.com/pubs/semnet.htm