Origins
When Peter Mark Roget published his Thesaurus of English Words and Phrases in 1852, he chose his title with exquisite care. The word 'thesaurus' means 'treasury' or 'storehouse,' from Latin 'thesaurus,' borrowed from Greek 'thesauros' (θησαυρός) — a treasure, a store, a place where valuable things are kept. Roget's ambition was precisely this: to build a treasury of the English language, organized not alphabetically like a dictionary but conceptually, grouping words by the ideas they express.
The Greek 'thesauros' has a disputed deeper etymology. Some scholars connect it to the verb 'tithemi' (to place, to put), suggesting a thesaurus is literally a 'place where things are put down' — a repository. Others have proposed a pre-Greek substrate origin, noting that the word does not follow typical Greek word-formation patterns. What is certain is that in ancient Greek, a 'thesauros' could be a treasure chamber, a strongbox, a storehouse for grain, or — most famously — a treasury building at a religious sanctuary. The treasuries at Delphi, where Greek city-states built elaborate structures to house their offerings to Apollo, were called 'thesauroi.' These were buildings designed to impress, and the word carried connotations of both wealth and display.
Latin borrowed 'thesaurus' wholesale, and the word entered the vocabulary of the Roman world with the same range of meanings. The Vulgate Bible uses 'thesaurus' in Matthew 6:21 — 'where your treasure is, there will your heart be also' — and early Christian writers extended the metaphor: a thesaurus could be a storehouse of knowledge, wisdom, or divine grace. The idea that knowledge could be stored and organized like treasure was not merely metaphorical but deeply influential. Medieval encyclopedias were sometimes titled 'Thesaurus' — treasuries of learning compiled for the benefit of future generations.
Latin Roots
In English, 'thesaurus' first appeared in the 1820s in its Latin sense of 'treasury' or 'storehouse.' But it was Roget who permanently altered the word's meaning. Roget was a physician, mathematician, chess player, and obsessive classifier — a man who found deep comfort in organizing the world into categories. He began compiling his word-lists in 1805, at the age of 26, and worked on them for nearly half a century before publication. His system divided the entire realm of human thought into six major classes (Abstract Relations, Space, Matter, Intellect, Volition, and Affective Faculties), each subdivided into increasingly specific categories. Under each category, he listed words and phrases expressing related concepts.
The book was an immediate and enduring success. It has never been out of print in over 170 years. The word 'thesaurus' itself, thanks entirely to Roget, shifted from meaning 'any treasury' to meaning specifically 'a reference book of synonyms.' Today, when English speakers say 'thesaurus,' they almost always mean a synonym dictionary, and many do not know that the word means 'treasure' at all. Roget's book so thoroughly claimed the word that its older meaning was effectively buried.
The same Greek root appears in several English words. 'Treasure' itself comes through Old French 'tresor' from the same Latin 'thesaurus.' The connection is direct: a treasure and a thesaurus are etymologically identical — one is a hoard of gold, the other a hoard of words. The modern Greek word 'thisavros' (θησαυρός) still means 'treasure' in everyday speech, and Greek speakers find it amusing that English uses their word for gold to describe a book of synonyms.
Spelling and Pronunciation
In computational linguistics, 'thesaurus' has taken on yet another meaning: a structured vocabulary that maps relationships between terms, used in information retrieval and natural language processing. WordNet, developed at Princeton, is perhaps the most famous computational thesaurus. Roget would have recognized the ambition — it is, after all, exactly what he spent his life building: a map of how words relate to one another, a treasury of language organized by meaning rather than by spelling.