WordNet was created and is being maintained at the Cognitive Science Laboratory of Princeton University under the direction of Psychology Professor George A. Miller. Development began in 1985. Over the years, the project received about $3 million of funding, mainly from government agencies interested in Machine translation.
WordNet distinguishes between nouns, verbs, adjectives and adverbs on the assumption that these are stored differently in the human Brain. Every synset contains a group of synonymous words or collocations (a collocation is a sequence of words that go together to form a specific meaning, such as "car pool"); words typically participate in several synsets. The meaning of the synsets is further clarified with short defining glosses. A typical example synset with gloss is:
Every synset is connected to other synsets via a number of relations. These relation vary based on the type of word:
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 several sample texts all words were semantically tagged with the corresponding synset, and then it was counted how often a word appeared in a specific sense.
The database's interface is able to deduce the root form of a Word from the user's input; only the root form is stored in the database.
dog, domestic dog, Canis familiaris
At the top level, these hierarchies are organized in to 25 primitive groups for nouns, and 15 for verbs. These groups form lexicographic files at maintenance level.
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 of lexicographic files, do not apply here the same way they do for nouns and verbs.
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, but modifying and maintaining the database is nonetheless difficult.
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.
The hypernym/hyponym relationships among the noun synsets can be used as an ontology in the Computer science sense. The SUMO upper ontology has produced a mapping from the WordNet synsets for nouns and verbs to SUMO classes. The OpenCyc upper ontology is also linked to WordNet. WordNet was the primary source for constructing the lower classes of the SENSUS ontology.
FrameNet is a similar project. 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 properites associated with it.