10 Captivating Insights About the Thesaurus for National Thesaurus Day

Writers frequently turn to a thesaurus to enrich their vocabulary and refine their writing. But using a thesaurus to find synonyms and antonyms is helpful for anyone—whether you're a writer or not—looking for more vivid and precise words to convey thoughts and ideas. In honor of Thesaurus Day on January 18, we’re sharing 10 fascinating facts about this invaluable tool.

1. The word Thesaurus is derived from the Greek term for treasure.

While many word lovers regard the thesaurus as a goldmine of language, the term thesaurus itself translates to "treasure." It comes from the Greek word thésauros, meaning a repository of valuable items, or simply, a treasure.

2. The plural form of thesaurus can be either thesauruses or thesauri.

What’s the correct way to refer to multiple octopus? People use variations like octopuses, octopi, and octopodes. Similarly, many people face confusion when determining the proper plural form of the word thesaurus. While thesauri is technically accurate due to its Latin suffix attached to the Latin word thēsaurus, both thesauri and thesauruses are widely accepted today.

3. The earliest thesauruses were essentially just dictionaries.

If you asked a French scholar in the 16th century to show you his thesaurus, he would happily offer you his dictionary. In the early 1530s, French printer Robert Estienne published the Thesaurus Linguae Latinae, an extensive Latin dictionary containing words found in Latin texts over an immense historical period. Then, in 1572, Estienne’s son Henri released the Thesaurus Linguae Graecae, a dictionary of Greek words. While these works were called thesauruses, they were, in reality, dictionaries with alphabetical word listings and their definitions.

4. The first known book of synonyms was written by a Greek historian.

Philo of Byblos, a Greek historian and grammarian, authored On Synonyms, a work considered to be the first ancient thesaurus. Believed to have been written in the late 1st or early 2nd century CE, this book catalogs Greek words with similar meanings. Unfortunately, much of On Synonyms has been lost to time, as no copies of the work have survived.

5. An early Sanskrit thesaurus was crafted in the form of a poem.

In the 4th century CE, the Indian poet and grammarian Amara Sinha composed The Amarakosha, a thesaurus of Sanskrit vocabulary. Instead of merely listing similar words, Amara Sinha transformed his thesaurus into a poetic work. Organized into three sections—divine, earthly, and everyday terms—The Amarakosha features verses designed to help readers easily memorize words. This thesaurus remains the oldest surviving book of its kind.

6. The first modern thesaurus was created by a British doctor.

Peter Mark Roget, the British doctor, is credited with creating the first modern thesaurus. In 1805, he started compiling a list of words organized by meaning and grouped them thematically. After retiring from his medical career in 1852, Roget published his seminal work, Thesaurus of English words and phrases; so classified and arranged as to facilitate the expression of ideas and assist in literary composition. Roget’s Thesaurus remains a commercial success and a widely used reference today. In fact, we celebrate Thesaurus Day on January 18, the birthday of Roget, born in 1779.

7. The thesaurus has an unexpected connection to a mathematical instrument.

The divide between "word enthusiasts" and "number enthusiasts" runs deep. Many mathematicians may shy away from thesauruses, while book lovers might avoid calculators, but the thesaurus is actually connected to a mathematical device. Around 1815, Roget invented the log-log slide rule, a ruler-like instrument designed to help users easily compute roots and exponents. While Roget was compiling words for his thesaurus, he was simultaneously working on the log-log slide rule. A true Renaissance man.

8. The Oxford English Dictionary has its own historical thesaurus.

In 1965, a professor of English Language at Glasgow University proposed the creation of a historical thesaurus based on the entries in the Oxford English Dictionary. The project was a monumental task, involving contributors from around the world over the span of 44 years to gather and organize words. Published in 2009, the Historical Thesaurus to the Oxford English Dictionary comprises 800,000 words, arranged by theme and date. This comprehensive work traces words and synonyms from Old English to the present, allowing readers to explore when certain words were first used and how long they remained in circulation.

9. One artist transformed his passion for words into a series of thesaurus-themed paintings.

In 2014, the Jewish Museum in New York hosted a retrospective of conceptual artist Mel Bochner’s works. For years, Bochner had been incorporating words and synonyms into his art, which came to be known as his thesaurus paintings. These vibrant works displayed lists of synonyms in English and Yiddish. One of the most striking pieces, titled Die, caused veterans of the Vietnam and Iraq wars to weep upon viewing. The painting included words and phrases such as expire, perish, succumb, drop dead, croak, go belly up, pull the plug, and kick the bucket.

10. There's an urban thesaurus for all your slang synonym needs.

While Urban Dictionary is a great resource for understanding modern slang, what about finding a thesaurus for those slang words? Enter Urban Thesaurus. This site, not associated with Urban Dictionary, catalogs millions of slang expressions from various dictionaries and analyzes how frequently terms are used together. For instance, typing in the word money brings up a quirky selection of synonyms like scrilla, cheddar, mulah, coin, and bling.