A word, phrase, or sentence is ambiguous if it has more than one meaning. The word 'light', for example, can mean not very heavy or not very dark. Words like 'light', 'note', 'bear' and 'over' are lexically ambiguous. They induce ambiguity in phrases or sentences in which they occur, such as 'light suit' and 'The duchess can't bear children'. However, phrases and sentences can be ambiguous even if none of their constituents is. The phrase 'porcelain egg container' is structurally ambiguous, as is the sentence 'The police shot the rioters with guns'. Ambiguity can have both a lexical and a structural basis, as with sentences like 'I left her behind for you' and 'He saw her duck'.
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There are two types of ambiguity, lexical and structural. Lexical ambiguity is by far the more common. Everyday examples include nouns like 'chip', 'pen' and 'suit', verbs like 'call', 'draw' and 'run', and adjectives like 'deep', 'dry' and 'hard'. There are various tests for ambiguity. One test is having two unrelated antonyms, as with 'hard', which has both 'soft' and 'easy' as opposites. Another is the conjunction reduction test. Consider the sentence, 'The tailor pressed one suit in his shop and one in the municipal court'. Evidence that the word 'suit' (not to mention 'press') is ambiguous is provided by the anomaly of the 'crossed interpretation' of the sentence, on which 'suit' is used to refer to an article of clothing and 'one' to a legal action.
The above examples of ambiguity are each a case of one word with more than one meaning. However, it is not always clear when we have only one word. The verb 'desert' and the noun 'dessert', which sound the same but are spelled differently, count as distinct words (they are homonyms). So do the noun 'bear' and the verb 'bear', even though they not only sound the same but are spelled the same. These examples may be clear cases of homonymy, but what about the noun 'respect' and the verb 'respect' or the preposition 'over' and the adjective 'over'? Are the members of these pairs homonyms or different forms of the same word? There is no general consensus on how to draw the line between cases of one ambiguous word and cases of two homonyous words. Perhaps the difference is ultimately arbitrary.
Sometimes one meaning of a word is derived from another. For example, the cognitive sense of 'see' seems derived from its visual sense. The sense of 'weigh' in 'He weighed the package' is derived from its sense in 'The package weighed two pounds'. Similarly, the transitive senses of 'burn', 'fly' and 'walk' are derived from their intransitive senses. Now it could be argued that in each of these cases the derived sense does not really qualify as a second meaning of the word but is actually the result of a lexical operation on the underived sense. This argument is plausible to the extent that the phenomenon is systematic and general, rather than peculiar to particular words. Lexical semantics has the task of identifying and characterizing such systematic phemena. It is also concerned to explain the rich and subtle semantic behavior of common and highly flexible words like the verbs 'do' and 'put' and the prepositions 'at', 'in' and 'to'. Each of these words has uses which are so numerous yet so closely related that they are often described as 'polysemous' rather than ambiguous.
Structural ambiguity occurs when a phrase or sentence has more than one underlying structure, such as the phrases 'Tibetan history teacher', 'a student of high moral principles' and 'short men and women', and the sentences 'The girl hit the boy with a book' and 'Visiting relatives can be boring'. These ambiguities are said to be...