Merriam-Webster’s Word of the Day for June 26, 2021 is:

thesaurus • thih-SOR-us  • noun

1 a : a book of words or of information about a particular field or set of concepts; especially : a book of words and their synonyms

b : a list of subject headings or descriptors usually with a cross-reference system for use in the organization of a collection of documents for reference and retrieval

2 : treasury, storehouse

Examples:

Seeking a more appropriate word than “secretive” to describe the uncommunicative subject of her piece, Mabel turned to her thesaurus and found one: “reticent.”

“Your objective is to communicate with your prospects and customers, not to wow them with just how darned smart you are.… Put the thesaurus and the five-syllable words away and write like you talk. Use short sentences and familiar words.” — Scott Flood, Inside Indiana Business, 12 May 2021

Did you know?

In the early 19th century, archaeologists borrowed the Latin word thesaurus to denote an ancient treasury, such as that in a temple. Soon after, the word was metaphorically applied to a book containing a treasury of words or information about a particular field. In 1852, the English scholar Peter Mark Roget published his Thesaurus of English Words and Phrases, in which he listed a treasury of semantically related words organized into numerous categories. This work led to the common acceptance of the term thesaurus to refer to “a book of words and their synonyms.” The word developed another meaning in the 1950s, when thesaurus began being used in the field of word processing to refer to a list of related terms used for indexing and retrieval.