A thesaurus lists synonyms and antonyms of index words. Peter Mark Roget published the first thesaurus, Thesaurus of English Words and Phrases in the 1850s, and current editions have become known simply as Roget's or Roget's Thesaurus. Most thesauruses (or thesauri) list index words alphabetically like a dictionary, but Roget organized the words topically.
The word thesaurus comes from the Greek θησαυρός, which means "treasury." Contrary to popular belief, the use of the -σαυρός suffix here is coincidental and unrelated to lizards.