Semantic lexicon

A semantic lexicon is a digital dictionary of words labeled with semantic classes so associations can be drawn between words that have not previously been encountered. Semantic lexicons are built upon semantic networks, which represent the semantic relations between words. The difference between a semantic lexicon and a semantic network is that a semantic lexicon has definitions for each word, or a "gloss".

Semantic lexicons are made up of lexical entries. These entries are not orthographic, but semantic, eliminating issues of homonymy and polysemy. These lexical entries are interconnected with semantic relations, such as hyperonymy, hyponymy, meronymy, or troponymy. Synonymous entries are grouped together in what the Princeton WordNet calls "synsets" Most semantic lexicons are made up of four different "sub-nets": nouns, verbs, adjectives, and adverbs, though some researchers have taken steps to add an "artificial node" interconnecting the sub-nets.

Nouns are ordered into a taxonomy. That is, they are structured into a hierarchy, with the broadest and most encompassing verb at the top, such as "thing", with the nouns becoming more and more specific the further they are form the top. The very top noun in a semantic lexicon is called a unique beginner. The most specific nouns (those that do not have any subordinates), are terminal nodes.

Another interesting attribute semantic lexicons have is that they distinguish between types and instances. The difference being that a type of something has characteristics of said thing, and an instance of something is an example of said thing. For example, a Rhodesian Ridgeback is a type of dog, but Dave Grohl is an instance of a musician. He is not a type of musician, because there are no others like him and even adapting all of his characteristics won’t make someone “a Dave Grohl”. Instances are always terminal nodes because they are solitary and don’t have other words or ontological categories belonging to them.

Semantic lexicons also address meronymy, which is a “part-to-whole” relationship, such as keys are part of a laptop. The necessary attributes that define a specific entry are also necessarily present in that entry’s hyponym. So, if a computer has keys, and a laptop is a type of computer, then a laptop must have keys. However, there are many instances where this distinction can become vague. A good example of this is the item chair. Most would define a chair as having legs and a seat (as in the part one sits on). However, there are some very “artistic” and “modern” chairs in overpriced boutiques that do not have legs at all. Beanbags also do not have legs, but few would argue that they aren't chairs. Questions like this are the core questions that drive research and work in the fields of taxonomy and ontology.

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