The common name of Peter Mark Roget’s reference book of synonyms is a thesaurus.
Roget’s work was first published in 1852 under the much longer title Thesaurus of English Words and Phrases, Classified and Arranged so as to Facilitate the Expression of Ideas and Assist in Literary Composition. The word thesaurus comes from Greek and Latin roots meaning a storehouse or treasury. In modern English, it usually means a reference work that groups words by related meanings.
Roget was a British physician, natural theologian, and lexicographer. He began organizing words by meaning in 1805, partly to help himself find precise expressions. His system arranged concepts into broad classes, divisions, and smaller groups rather than listing words only in alphabetical order. That structure lets readers move from an idea to related vocabulary.
A thesaurus is not exactly the same as a dictionary. A dictionary primarily explains meanings, pronunciation, spelling, and usage, while a thesaurus helps users find synonyms and related expressions. The entries in Roget’s system are not always perfect substitutes: words can share a general meaning while differing in tone, strength, or context. Roget’s Thesaurus became influential, and later editions were expanded by his descendants.