ROGET'S THESAURUS, full title Roget's Thesaurus of English Words and Phrases. A work of REFERENCE by the English physician, taxonomist, librarian, and inventor Peter Mark Roget (1779–1869), published in 1852 by Longman. Its aim was to supply, with respect to the English language, a desideratum hitherto unsupplied in any language; namely, a collection of the words it contains and of the idiomatic combinations peculiar to it, arranged, not in alphabetical order as they are in a Dictionary, but according to the ideas which they express' (original introduction). The Thesaurus has been revised many times, has sold over 30m copies worldwide in different editions, and has become an institution of the language. The words in Roget are arranged in listed sets like the genera and species of biology. The intention has not been to define or discriminate them, but to arrange them in synonymous and antonymous groups; it serves as both a word-finder and a prompter of the memory regarding words one knows but could not recall to mind.

The Thesaurus was sustained by the Roget family for three generations: Roget himself brought out several revisions before his death in 1869, after which his son John Lewis Roget, a lawyer, continued the work (adding a word-index, which greatly aided use of the book, and a major revision in 1879) until his death in 1908, when his son Samuel Romilly Roget, an engineer, took over, providing a major revision in 1936. During Samuel's time as editor, the boom for crossword puzzles developed, creating a demand for the word-finder that neither his father nor grandfather could have predicted. He sold the rights to Longman in 1952, the year before he died. Since then there have been three revisions: by Robert A. Dutch (1962), Susan M. Lloyd (1982), and Betty Kirkpatrick (1987). There have been many changes between 1852 and 1987, but Roget's basic structure survives in the Longman work, whose latest edition is entitled The Original Roget's Theasaurus to disintinguish it from various adaptations by other publishers, especially alphabetic versions in the US.

The classification of the Thesaurus is hierarchical, with six major headings: Abstract Relations, Space, Matter, Intellect, Volition, Affections. In the 1982 and 1987 editions, Affections has been replaced by Emotion, religion, and morality. Each of these headings is further divided, Abstract Relations for example into: Existence, Relation, Quantity, Order, Number, Time, Change, Causation. Each subheading is further subdivided, Existence for example into: Existence, Inexistence, Substantiality, Unsubstantiality, Intrinsicality, Extrinsicality, State, Circumstance. At this third level come the specific sets of words, among which are italicized keywords preceded by numbers. These keywords, which were introduced by Dutch, serve as cross-references to sets elsewhere in the book. The following is the content and layout of part of set no. 1 in the 1987 edition:
existence, being, entity; absolute being, the absolute 965 divineness; aseity, self-existence; monad, a being, an entity, ens, essence, quiddity; Platonic idea, universal; subsistence 360 life; survival, eternity 115 perpetuity; preexistence 119 priority; this life 121 present time; existence in space, prevalence 189 presence; entelechy, realization, becoming, evolution 147 conversion; creation 164 production; potentiality 469 possibility; ontology, metaphysics; realism, materialism, idealism, existentialism 449 philosophy.

In addition to such thematic sets, which make up the bulk of the present-day book, a detailed index shows where a listed word may be found: for example, existence is shown as appearing not only in existence 1, but also in presence 189, materiality 319, and life 360. Longman claim that over 1.25m words are covered in the 1987 Kirkpatrick edition. Also in 1987, Bloomsbury (London) brought out a facsimile edition of the original Thesaurus, with an introduction by Laurence Urdang. See LEXICOGRAPHY, THESAURUS.