Many English learners know a thesaurus as a great tool for finding synonyms, but things get tricky when you try to make it plural. Should it be thesauruses or thesauri? Let’s clear up the confusion with simple explanations, examples, and tips to help you use the right form every time.
Definition and Plural of Thesaurus
What Does “Thesaurus” Mean?
A thesaurus is a reference book or tool that lists words grouped by meaning — such as synonyms and antonyms. In modern English, it can refer to either a printed book or an online database.
Definition:
👉 Thesaurus = a book or resource that shows synonyms and related words.
Examples:
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I used a thesaurus to find another word for “happy.”
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The thesaurus is helpful for improving vocabulary.
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She keeps a thesaurus beside her dictionary.
What Is the Plural of “Thesaurus”?
The word thesaurus has two correct plural forms: thesauruses and thesauri.
| Singular | Plural (Modern English) | Plural (Latin form) |
|---|---|---|
| thesaurus | thesauruses | thesauri |
✅ Both are correct, but “thesauruses” is more common in everyday English, while “thesauri” is used in academic or formal writing.
Examples:
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Modern thesauruses are available online. (common)
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Ancient scholars compiled several thesauri of knowledge. (formal)
Pronunciation Guide
| Word | IPA Pronunciation | How It Sounds |
|---|---|---|
| thesaurus | /θɪˈsɔː.rəs/ | “thi-SOR-us” |
| thesauruses | /θɪˈsɔː.rə.sɪz/ | “thi-SOR-uh-siz” |
| thesauri | /θɪˈsɔː.raɪ/ | “thi-SOR-rye” |
Examples of “Thesaurus” and Its Plural Forms
Singular: “Thesaurus”
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A thesaurus helps writers find better words.
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The thesaurus lists many synonyms for “good.”
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She bought a new thesaurus for her English class.
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The thesaurus is next to the dictionary on the shelf.
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Using a thesaurus can make your writing more varied.
Plural: “Thesauruses” (Common English Form)
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Online thesauruses are more convenient than printed ones.
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There are several English thesauruses available for free.
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The teacher recommended three different thesauruses.
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Many modern thesauruses include idioms and phrasal verbs.
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Linguists created digital thesauruses for various languages.
Plural: “Thesauri” (Latin Form)
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Ancient scholars compiled multiple thesauri of Greek words.
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The library contains several thesauri on linguistics.
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Researchers compared old and modern thesauri.
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Some dictionaries also function as thesauri.
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The project combined data from five thesauri.
Common Mistakes English Learners Make
❌ Mistake 1: Writing “thesauris” or “thesauris’s”
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Incorrect: The book lists different thesauris.
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✅ Correct: The book lists different thesauruses or thesauri.
❌ Mistake 2: Thinking only one plural is correct
Both thesauruses and thesauri are accepted — choose based on context.
❌ Mistake 3: Spelling confusion
Remember that “thesaurus” ends in -us, not -ous.
Practice Exercises
Fill in the blanks with thesaurus, thesauruses, or thesauri.
Exercise 1
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I always use a ______ when writing essays.
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Modern ______ can be found online.
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The library has several ______ for students to use.
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Ancient scholars created many ______ of Latin words.
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Every writer should have a good ______ on their desk.
✅ Answer Key:
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thesaurus
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thesauruses
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thesauruses
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thesauri
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thesaurus
Exercise 2: Choose the correct sentence.
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a) I found two thesauruses on sale. ✅
b) I found two thesauris on sale. -
a) The ancient thesauri were written by Greek scholars. ✅
b) The ancient thesauruses were written by Greek scholars. (also acceptable) -
a) There are many online thesauruses today. ✅
b) There are many online thesauris today. -
a) A thesaurus is different from a dictionary. ✅
b) A thesauruses is different from a dictionary. -
a) Linguists compared five thesauri in their research. ✅
b) Linguists compared five thesauris in their research.