noun (plural thesauri /-rʌɪ/ or thesauruses)
late 16th century: via Latin from Greek thēsauros 'storehouse, treasure'. The original sense 'dictionary or encyclopedia' was narrowed to the current meaning by the publication of Roget's Thesaurus of English Words and Phrases (1852)
Spell thesaurus with -au- in the middle. The plural can be spelled either thesauruses or thesauri (like the original Latin).