θησαυρός

The Greek noun thēsaurós meant a treasury, a storehouse, a place where valuable things were deposited for safekeeping. The word's etymology is uncertain — possibly related to tithénai (to put, to place) or tithenai's root, suggesting 'a place where things are put,' or alternatively from a pre-Greek substrate word. In ancient Greece, thēsaurós referred most concretely to the small treasury buildings constructed by individual Greek city-states at Delphi and Olympia to house their offerings to the gods — the Treasury of Athens, the Treasury of Siphnos. These were small, elegant buildings, like private vaults, and several still stand at Delphi. The metaphorical extension to any treasury of valuable things was natural and ancient.

In Byzantine and medieval scholarship, thēsauros (in its Latin form thesaurus) was used for encyclopedic compilations of knowledge — treasuries of learning. The lexicographer Henri Estienne (Henricus Stephanus) published his monumental Thesaurus Graecae Linguae in 1572, a comprehensive Greek-Latin dictionary that organized the entire vocabulary of classical Greek — truly a treasury of language. The title established thesaurus as the appropriate name for a comprehensive lexical collection, and the usage persisted in scholarly bibliography.

Peter Mark Roget, a British physician, had compiled a private list of words organized by concept — not alphabetically but thematically, grouping words by what they meant rather than how they were spelled — as early as 1805. He published it in 1852 as Roget's Thesaurus of English Words and Phrases, borrowing the title's scholarly prestige and its core metaphor: here was a treasury of words, organized so the writer could retrieve exactly the right one. Roget's arrangement was unique — conceptual categories rather than alphabetical order — and his specific structure influenced how thesauruses were organized for a century. The word thesaurus now means specifically a reference book of synonyms and related words, and the name Thesaurus has become so associated with Roget's specific publication that it functions almost as a brand, even when applied to competitors.