• Nouns
  o hypernyms: Y is a hypernym of X if every X is a (kind of) Y
  o hyponyms: Y is a hyponym of X if every Y is a (kind of) X
  o coordinate terms: Y is a coordinate term of X if X and Y share a hypernym
  o holonym: Y is a holonym of X if X is a part of Y
  o meronym: Y is a meronym of X if Y is a part of X
• Verbs
  o hypernym: the verb Y is a hypernym of the verb X if the activity X is a (kind of) Y (travel to movement)
  o troponym: the verb Y is a troponym of the verb X if the activity Y is doing X in some manner (lisp to talk)
  o entailment: the verb Y is entailed by X if by doing X you must be doing Y (sleeping by snoring)
  o coordinate terms: those verbs sharing a common hypernym
• Adjectives
  o related nouns
  o participle of verb
• Adverbs
  o root adjectives

While semantic relations apply to all members of a synset because they share a meaning but are all mutually synonyms, words can also be connected to other words through lexical relations, including antonyms (opposites of each other) and derivationally related, as well.

WordNet also provides the polysemy count of a word: the number of synsets that contain the word. If a word participates in several synsets (i.e. has several senses) then typically some senses are much more common than others. WordNet quantifies this by the frequency score: in which several sample texts have all words semantically tagged with the corresponding synset, and then a count provided indicating how often a word appears in a specific sense.

The morphology functions of the software distributed with the database try to deduce the lemma or root form of a word from the user's input; only the root form is stored in the database unless it has irregular inflected forms.

Knowledge structure

Both nouns and verbs are organized into hierarchies, defined by hypernym or IS A relationships. For instance, the first sense of the word dog would have the following hypernym hierarchy; the words at the same level are synonyms of each other: some sense of dog is synonymous with some other senses of domestic dog and Canis familiaris, and so on. Each set of synonyms (synset), has a unique index and shares its properties, such as a gloss (or dictionary) definition.

dog, domestic dog, Canis familiaris
=> canine, canid
=> carnivore
=> placental, placental mammal, eutherian, eutherian mammal
=> mammal
=> vertebrate, craniate
=> chordate
=> animal, animate being, beast, brute, creature, fauna
=> ...

At the top level, these hierarchies are organized into base types, 25 primitive groups for nouns, and 15 for verbs. These groups form lexicographic files at a maintenance level. These primitive groups are connected to an abstract root node that have, for some time, been assumed by various applications that use WordNet.

In the case of adjectives, the organization is different. Two opposite 'head' senses work as binary poles, while 'satellite' synonyms connect to each of the heads via synonymy relations. Thus, the hierarchies, and the concept involved with lexicographic files, do not apply here the same way they do for nouns and verbs.

The network of nouns is far deeper than that of the other parts of speech. Verbs have a far bushier structure, and adjectives are organized into many distinct clusters. Adverbs are defined in terms of the adjectives they are derived from, and thus inherit their structure from that of the adjectives.

Psychological justification

The goal of WordNet was to develop a system that would be consistent with the knowledge acquired over the years about how human beings process language. Anomic aphasia, for example, creates a condition that seems to selectively encumber individuals' ability to name objects; this makes the decision to partition the parts of speech into distinct hierarchies more of a principled decision than an arbitrary one.

In the case of hyponymy, psychological experiments revealed that individuals can access properties of nouns more quickly depending on when a characteristic becomes a defining property. That is, individuals can quickly verify that canaries can sing because a canary is a songbird (only one level of hyponymy), but requires slightly more time to verify that canaries can fly (two levels of hyponymy) and even more time to verify canaries have skin (multiple levels of hyponymy). This suggests that we too store semantic information in a way that is much like WordNet, because we only retain the most specific information needed to differentiate one particular concept from similar concepts.

WordNet as an ontology

The hypernym / hyponym relationships among the noun synsets can be interpreted as specialization relations between conceptual categories. In other words, WordNet can be interpreted and used as a lexical ontology in the computer science sense. However, such an ontology should normally be corrected before being used since it contains hundreds of basic semantic inconsistencies such as (i) the existence of common specializations for exclusive categories and (ii) redundancies in the specialization hierarchy. Furthermore, transforming WordNet into a lexical ontology usable for knowledge representation should normally also involve (i) distinguishing the specialization relations into subtypeOf and instanceOf relations, and (ii) associating intuitive unique identifiers to each category. Although such corrections and transformations have been performed and documented as part of the integration of WordNet 1.7 into the cooperatively updatable knowledge base of WebKB-2, most projects claiming to re-use WordNet for knowledge-based applications (typically, knowledge-oriented information retrieval) simply re-use it as such.

A prominent example of using WordNet, as it is, as an ontology is to determine the similarity between words. Various algorithms have been proposed, and these include considering the distance between the conceptual categories of these words, as well as considering the hierarchical structure of the WordNet ontology. A number of these WordNet-based word similarity algorithms are implemented in a Perl package called WordNet::Similarity.

See the related projects section for more.

Limitations

Unlike other dictionaries, WordNet does not include information about etymology, pronunciation and the forms of irregular verbs and contains only limited information about usage.

The actual lexicographical and semantical information is maintained in lexicographer files, which are then processed by a tool called grind to produce the distributed database. Both grind and the lexicographer files are freely available in a separate distribution, but modifying and maintaining the database requires expertise.

Though WordNet contains a sufficient wide range of common words, it does not cover special domain vocabulary. Since it is primarily designed to act as an underlying database for different applications, those applications cannot be used in specific domains that are not covered by WordNet.

Interfaces

The Jawbone project provides a Java API to the WordNet 2.1 and 3.0 data. The source code is released under the MIT license.

The Natural Language Toolkit provides a Python API to the WordNet 3.0.

Related projects

A project at Brown University started by Jeff Stibel, James A. Anderson, Steve Reiss and others called Applied Cognition Lab created a disambiguator using WordNet in 1998.[2] The project later morphed into a company called Simpli, which is now owned by ValueClick. George Miller joined the Company as a member of the Advisory Board. Simpli built an Internet search engine that utilized a knowledgebase principally based on WordNet to disambiguate and expand keywords and synsets to help retrieve information online. WordNet was expanded upon to add increased dimensionality, such as intentionality (used for x), people (Britney Spears) and colloquial terminology more relevant to Internet search (i.e., blogging, ecommerce). Neural network algorithms searched the expanded WordNet for related terms to disambiguate search keywords (Java, in the sense of coffee) and expand the search synset (Coffee, Drink, Joe) to improve search engine results.[3] Before the company was acquired, it performed searches across search engines such as Google, Yahoo!, Ask.com and others.[4]

The project EuroWordNet has produced WordNets for several European languages and linked them together; these are not freely available however. The Global Wordnet project attempts to coordinate the production and linking of wordnets for all languages. Oxford University Press, the publishers of the Oxford English Dictionary have voiced plans to produce their own online WordNet.

The eXtended WordNet is a project at the University of Texas at Dallas which aims to improve WordNet by semantically parsing the glosses, thus making the information contained in these definitions available for automatic knowledge processing systems. It is also freely available under a license similar to WordNet's.

The GCIDE project produces a dictionary by combining a public domain Webster's Dictionary from 1913 with some WordNet definitions and material provided by volunteers. It is released under the copyleft license GPL.

WordNet is also commonly re-used via mappings between the WordNet categories and the categories from other ontologies. Most often, only the top-level categories of WordNet are mapped. However, the authors of the SUMO ontology have produced a mapping between all of the WordNet synsets, (including nouns, verbs, adjectives and adverbs), and SUMO classes. The most recent addition of the mappings provides links to all of the more specific terms in the MId-Level Ontology (MILO), which extends SUMO. The OpenCyc upper ontology is also linked to some of WordNet.

In most works that claim to have integrated WordNet into other ontologies, the content of WordNet has not simply been corrected when semantic problems have been encountered; instead, WordNet has been used as an inspiration source but heavily re-interpreted and updated whenever suitable. This was the case when, for example, the top-level ontology of WordNet was re-structured according to the OntoClean based approach or when WordNet was used as a primary source for constructing the lower classes of the SENSUS ontology.

FrameNet is a project similar to WordNet. It consists of a lexicon which is based on annotating over 100,000 sentences with their semantic properties. the unit in focus is the lexical frame, a type of state or event together with the properties associated with it.

An independent project titled wordNet with an initial lowercase w is an ongoing project to links words and phrases via a custom Web crawler.