1. Ontology and the Lexicon

A lexicon is a list of words in a language—a vocabulary—along with some knowledge of how each word is used. A lexicon may be general or domain-specific; we might have, for example, a lexicon of several thousand common words of English or German, or a lexicon of the technical terms of dentistry in some language. The words that are of interest are usually open-class or content words, such as nouns, verbs, and adjectives, rather than closed-class or grammatical function words, such as articles, pronouns, and prepositions, whose behaviour is more tightly bound to the grammar of the language. A lexicon may also include multi-word expressions such as fixed phrases (by and large), phrasal verbs (tear apart), and other common expressions (merry Christmas!; teach someone's grandmother to suck eggs; Elvis has left the building). Each word or phrase in a lexicon is described in a lexical entry; exactly what is included in each entry depends on the purpose of the particular lexicon. The details that are given (to be discussed further in sections 1.2.1 and 1.3.2 below) may include any of its properties of spelling or sound, grammatical behaviour, meaning, or use, and the nature of its relationships with other words. A lexical entry is therefore a potentially large record specifying many aspects of the linguistic behaviour and meaning of a word. Hence a lexicon can be viewed as an index that maps from the written form of a word to information about that word. This is not a one-to-one correspondence, however. Words that occur in more than one syntactic category will usually have a separate entry for each category; for example, flap would have one entry as a noun and another as a verb. Separate entries are usually also appropriate for each of the senses of a homonym—a word that has more than one unrelated sense even within a single syntactic category; for example, the noun pen would have distinct entries for the senses writing instrument, animal enclosure, and swan. Polysemy— related or overlapping senses—is a more-complex situation; sometimes the senses may be discrete enough that we can treat them as distinct: for example, window as both opening in wall and glass pane in opening in wall (fall through the window; break the window). But this is not always so; the word open, for example, has many overlapping senses concerning unfolding, expanding, revealing, moving to an open position, making openings …