What's in a Thesaurus?

We first describe four varieties of thesaurus: (1) Roget-style, produced to help people find synonyms when they are writing; (2) WordNet and EuroWordNet; (3) thesauruses produced (manually) to support information retrieval systems; and (4) thesauruses produced automatically from corpora. We then contrast thesauruses and dictionaries, and present a small experiment in which we look at polysemy in relation to thesaurus structure. It has sometimes been assumed that different dictionary senses for a word that are close in meaning will be near neighbours in the thesaurus. This hypothesis is explored, using as inputs the hierarchical structure of WordNet 1.5 and a mapping between WordNet senses and the senses of another dictionary. The experiment shows that pairs of ‘lexicographically close’ meanings are frequently found in different parts of the hierarchy. In the first part of the paper, we present different varieties of thesaurus. In the second part, we contrast thesaurus word senses with dictionary word senses and present a small experiment in which we explore whether ‘lexicographically close’ meanings are often close in the WordNet network.