A thesaurus is a reference which allows a person to look up synonyms, and sometimes antonyms, for words or phrases. It comes from the Greek thēsauros, which meant a treasury. The adverbial derivative is thesaural.
As with other word that have stayed largely unchanged from their origin in Greek or Latin, the Latin pluralization is correct, thesauri. However, as with other words that have been adopted into the English language for centuries, the English pluralization is also correct, thesauruses.
The actual use of each plural seems to be evenly divided between thesauri and thesauruses. One factor is the blatant difficulty of saying thesauruses, as well as the loyalists who will always prefer the Latin plural over the English.
Sundberg’s color thesaurus is comprised of a series of charts containing several variations of common colors like white, tan, yellow, orange, etc. [Small Business Trends].
Evens deals with issues on the use of thesaural relations for information retrieval (IR) applications. [Green, Bean, and Myaeng]
They are very involved in the community and local giving, including scholarships for high school and continuing education students, dictionaries and thesauri for Bisbee school children, donations to local non-profits like the Boys and Girls Club of Bisbee, and internationally through the Rotary International Foundation, as well as shelter for displaced families due to war or natural disaster via Shelter Boxes. [Sierra Vista Herald]
The Fed watchers, failed utterly by their thesauruses (synonyms for “considerable” can range from “reasonable” and “respectable” to “huge” and “astronomical”), have generally concluded that an increase remains very likely to take place next year. [The New Yorker]