National Thesaurus Day Date in the current year: January 18, 2024

A thesaurus, also known as a dictionary of synonyms or a synonym dictionary, is a type of dictionary where words are arranged by their meanings, allowing to find different words that have the same meaning as other words.

The word “thesaurus” comes from Latin thēsaurus, which in turn derives from ancient Greek θησαυρός (thēsaurós) that can be translated into English as “treasure”, “treasury”, or “storehouse”. Until the 19th century, the word “thesaurus” was synonymous with “dictionary” or “encyclopedia”. The first person to use it in its present-day meaning was Peter Mark Roget, the author of Roget’s Thesaurus.

It should be noted that synonym dictionaries existed before Roget’s time. For example, An Essay Towards a Real Character, and a Philosophical Language and Alphabetical Dictionary by John Wilkins, both published in 1668, can be considered early synonym dictionaries even though they don’t use the word “synonym”. However, it was Roget’s Thesaurus that became the first widely used synonym dictionary in the English-speaking world.

Peter Mark Roget was born on January 18, 1779. A trained physician, Roget had a career in medicine, but he was also interested in lexicography. Roget started to maintain a notebook of words grouped by meaning in 1805 and began working on what would become his Thesaurus full-time upon his retirement in 1840.

The first edition of Roget’s Thesaurus was published in 1852; it contained 15,000 words grouped into six classes, each composed of multiple divisions, which, in turn, contained sections and subsections. The thesaurus has been updated multiple times since its first publication, with the 8th edition (the most recent one as of 2024) containing 443,000 words. However, all editions keep true to Roget’s original classification system.

Of course, not all thesauruses use Roget’s system. Some use a hierarchical taxonomy of broader and narrower terms, while others simply list synonyms and antonyms alphabetically or in some other order. Most thesauruses, unlike other types of dictionaries, do not include word definitions

The origin of National Thesaurus Day is unclear, but there is no doubt that Roget and his thesaurus deserve to be celebrated. You can observe the holiday by flipping through a thesaurus and learning some new words, playing a synonym game with friends to expand your vocabulary, rewriting a famous story replacing as many words with synonyms as you can, and spreading the word about the holiday on social media with the hashtags #NationalThesaurusDay and #ThesaurusDay.

National Thesaurus Day should not be confused with Dictionary Day, sometimes referred to as National Dictionary Day. The latter is celebrated on October 16 to commemorate the birthday of Noah Webster, an American lexicographer whose name has become synonymous with “dictionary” in the US thanks to the Mirriam-Webster dictionary.