A thesaurus is a reference book that groups words that have similar meanings. A thesaurus may also include a section that groups words with nearly opposite meanings as well. The first group of words are called synonyms, and the latter are called antonyms.
In the original Greek, thesaurus meant "treasury" or "repository." Writers use a thesaurus to find words that are more unusual or more precise than other, more common words they might use. A thesaurus differs from a dictionary, which provides definitions and guidance on pronunciation. Roget's Thesaurus, published in 1852, is the earliest known thesaurus in English.
In the computer industry, the term thesaurus also refers to a searchable subject matter index of a particular field.