Polysemy is characterized as the phenomenon whereby a single word form is associated with two or several related senses. It is distinguished from monosemy, where one word form is associated with a single meaning, and homonymy, where a single word form is associated with two or several unrelated meanings. Although the distinctions between polysemy, monosemy, and homonymy may seem clear at an intuitive level, they have proven difficult to draw in practice. Polysemy proliferates in natural language: Virtually every word is polysemous to some extent. Still, the phenomenon has been largely ignored in the mainstream linguistics literature and in related disciplines such as philosophy of language. However, polysemy is a topic of relevance to linguistic and philosophical debates regarding lexical meaning representation, compositional semantics, and the semantics–pragmatics divide. Early accounts treated polysemy in terms of sense enumeration: each sense of a polysemous expression is represented individually in the lexicon, such that polysemy and homonymy were treated on a par. This approach has been strongly criticized on both theoretical and empirical grounds. Since at least the 1990s, most researchers converge on the hypothesis that the senses of at least many polysemous expressions derive from a single meaning representation, though the status of this representation is a matter of vivid debate: Are the lexical representations of polysemous expressions informationally poor and underspecified with respect to their different senses? Or do they have to be informationally rich in order to store and be able to generate all these polysemous senses? Alternatively, senses might be computed from a literal, primary meaning via semantic or pragmatic mechanisms such as coercion, modulation or ad hoc concept construction (including metaphorical and metonymic extension), mechanisms that apparently play a role also in explaining how polysemy arises and is implicated in lexical semantic change.