R. S. Ginzburg, S. S. Khidekel, G. Y. Knyazeva, A. A. Sankin a course in modern english

§ 25. Summary and Conclusions

1. Not only the sound-form but also the meaning of the word is changed in the course of the historical development of language. The factors causing semantic changes may be roughly subdivided into extra-linguistic and linguistic causes.

2. 
   Change of meaning is effected through association between the existing meaning and the new. This association is generally based on the similarity of meaning (metaphor) or on the contiguity of meaning (metonymy).
   
3. 
   Semantic changes in the denotational component may bring about the extension or the restriction of meaning. The change in the connotational component may result in the pejorative or ameliorative development of meaning.
   
4. 
   Causes, nature and result of semantic changes should be regarded as three essentially different but closely connected aspects of the same linguistic phenomenon.
   

MEANING AND POLYSEMY

So far we have been discussing the concept of meaning, different types of word-meanings and the changes they undergo in the course of the historical development of the English language. When analysing the word-meaning we observe, however, that words as a rule are not units of a single meaning. Monosemantic words, i.e. words having only one meaning are comparatively few in number, these are mainly scientific terms, such -as hydrogen, molecule and the like. The bulk of English words are polysemantic, that is to say possess more than one meaning. The actual number of meanings of the commonly used words ranges from five to about a hundred. In fact, the commoner the word the more meanings it has.