In this chapter we consider how the words of a language change. In previous chapters we have considered many changes that affect words. In Chapters 2 and 3 the discussion of sound change showed that the phonetic form of words can change, usually in a regular manner. In Chapters 4 and 5 we saw that the morphological composition of words can change as well. In Chapters 6 and 7 we saw that some words in specific constructions undergo grammaticalization and become grammatical morphemes. In Chapter 8 we saw that the particular constructions that words occur in can also change, which can affect the meaning of the words.
Here we consider additional types of change in words or the lexicon. First, we discuss the ways in which languages acquire new words in Section 9.2. Then we consider how words change their meanings, spreading this discussion over several sections: first in Section 9.3 we discuss the nature of the categories that constitute word meaning, and then we treat the mechanism by which words change their meaning over time. In Section 9.4 we discuss some general tendencies in lexical semantic change, and then in Section 9.5 we discuss the way words that are formed by derivational affixes lose their compositional meaning. Finally, Section 9.6 adds a few comments about how old words are lost from a language.