We think of words as the basic units of language. When a baby begins to
speak,It is clear also that words on their own, outside sentences, can be sorted
and classified in various ways. A comprehensive classification of English
words according to meaning is a thesaurus, such as Roget’s Thesaurus. But
the kind of conventional classification that we are likely to refer to most
often is a dictionary, in which words are listed according to their spelling
in alphabetical order