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The Web of Words

Brian Hayes

Synonyms, Hypernyms and Other Nyms

In WordNet the emphasis is less on words than on the relations between words. And among the various relations defined in the lexical graph, the most fundamental is synonymy. Words that mean more or less the same thing are grouped into synonym sets, or synsets, much as they are in a thesaurus. The synsets (rather than individual words) then become the basic nodes of the graph.

A finicky logophile might well argue that true synonyms do not exist—that no two words are exactly equivalent. The compilers of WordNet take a pragmatic position on this issue. They classify words as synonyms if there is some class of sentences where one word can take the place of another without substantially altering the meaning. Thus yell, shout and holler have distinguishable nuances of meaning, but in many sentences the words are interchangeable.

Figure 1. Small portion of the WordNet lexical graphClick to Enlarge Image

Most words have multiple meanings, and synonymy is really a relation between the individual senses of the words. The adjective light is an approximate synonym both of weightless and of pale, but weightless and pale are not themselves synonyms. Each sense of light has to be given its own synset. (In the current version of WordNet light has 26 adjective senses as well as 15 noun senses, six verb senses and one adverb sense.)

Synonymy is the glue that binds WordNet together. Nouns, verbs, adjectives and adverbs all have synonyms. Nevertheless, the structure of the lexical graph comes mainly from other kinds of relations, which are somewhat different for each part of speech.

For nouns the most important relations are hypernymy and hyponymy, which organize concepts into a treelike hierarchy. Hypernyms and hyponyms embody the "is-a" or "is-a-kind-of" relation: A horse is a mammal is an animal is an organism. Thus the word horse (or the sense of horse referring to a hoofed quadruped) is a hyponym of mammal, which in turn is a hyponym of animal, and so on. Hypernyms describe the same relation seen from the other end of the telescope: Animal is a hypernym of mammal and also of reptile, bird, fish, etc. The biological examples are apt here, since the construction of such taxonomic hierarchies is a specialty of the life sciences. In WordNet the sequence of hypernyms for horse captures much of the phylogenetic detail a biologist would want to see recorded: horse → equine → odd-toed ungulate → ungulate → placental mammal → mammal → vertebrate → chordate → animal → organism → entity.

Another relation among nouns is summed up in the phrase "has a" rather than "is a." This is the relation between parts and wholes; words that represent parts or members are called meronyms, and those that denote wholes or groups are holonyms. The distinction between "is a" and "has a" can be subtle, but there are examples that make it clear. Consider the noun meal: Its hyponyms are words such as breakfast, lunch and dinner, but its meronyms are appetizer, salad, dessert and so on.

The same kind of treelike organization can be imposed on verbs, although verb trees tend to be somewhat stunted and shrubby compared with noun trees. The relation analogous to hyponymy in nouns has been dubbed troponymy in verbs. For example, the intransitive verb walk is a troponym of go or move or locomote; in other words, walking is a way of moving. And walk in turn has troponyms such as shuffle, amble, swagger and march. For some verbs there is also a relation analogous to meronymy in nouns, defining the component parts of an action. The verb step fills this role for walk, since walking entails taking steps.

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