"Roget's Thesaurus of English Words and Phrases" by Peter Mark Roget is a thesaurus first created in 1805 and published in 1852. This groundbreaking reference work organizes words not alphabetically but through a unique classification system of six primary classes, with branches for meaning clusters and semantically linked terms. Rather than simple synonyms, it presents words as shades of meaning within conceptual spectrums. The original edition contained 15,000 words; modern editions now include over 443,000, maintaining Roget's original organizational framework while expanding to reflect contemporary language.