A thesaurus is a book of synonyms and antonyms; a dictionary of alternative expressions; a treasury of linguistic equivalents; a useful means of discovering alternative ways of saying exactly the same thing.

The first thesaurus was compiled by the English physician, Peter Roget, and was published in 1852. Roget chose to group his words not alphabetically, but in sequences of closely related ideas. Thus, if an individual was unable to find a suitable synonym in one of the listed categories, he would be able to find it nearby. A more conventional index was published at the back of the book.

Recent alternative versions of the thesaurus have favoured an alphabetical approach throughout.

Roget oversaw some 25 editions of his book, before his death in 1869. New editions have been published regularly ever since.