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Dictionary, thesaurus or glossary: which one you actually need.
A dictionary tells you what a word means. A thesaurus gives you other words that mean roughly the same thing. A glossary explains the special terms of one subject, usually at the back of the book that uses them. Use the dictionary when you meet a word you don’t know, the thesaurus when you know what you mean and want a better word for it, and the glossary when the field has its own language. Here is what each does, where each fails, and what a gloss is.
The three, side by side
| Dictionary | Thesaurus | Glossary | |
|---|---|---|---|
| What it contains | Words in alphabetical order with meanings, pronunciation, part of speech, origin, sometimes usage notes | Words grouped by meaning, each with a list of synonyms and often antonyms | The special terms of one subject or one book, with short definitions |
| The question it answers | What does this word mean? | What is another word for this? | What does this word mean here, in this field? |
| Where it fails | Defines the word in other words you may also not know | Lists words that overlap as if they were interchangeable | Covers only its own subject |
| Use it when | You meet a word you don’t know | You know what you mean and want a better word | You are reading a textbook, a contract, a manual |
The dictionary
From the Latin dictio, a saying. A dictionary is a list of words with what they mean, and since Samuel Johnson’s of 1755 it has also carried how to say them, where they come from and how they are used. Its weakness is built in: it explains words with words. Ineffable: incapable of being expressed in words is accurate and no help to a nine-year-old. That is why some dictionaries for learners use a controlled vocabulary of a few thousand words for every definition, and why Gloss, which we make, writes the explanation in the style you choose and leads with a sentence you can picture. The best dictionary apps compares the current ones.
The thesaurus
From the Greek thesauros, a treasury. Peter Mark Roget published his in 1852 as a tool for finding the word you were reaching for, not for decorating prose, and the misuse began almost at once. A thesaurus lists happy, content, elated, ecstatic together because they overlap; they are not the same word, and a sentence that swaps one for another changes its meaning. The fix is a thesaurus that says how each word differs, which is what our another word for pages do for fifty everyday words, and what the Synonyms tab in Gloss does for every word. Words to use instead of “very” is the same idea applied to the word people most want to replace.
The glossary
From the Greek glossa, tongue, which came to mean a foreign or difficult word, then the explanation of one. A gloss is a short explanation of a hard word, written between the lines or in the margin; medieval readers glossed their Latin texts in their own languages, and those glosses are some of the oldest writing we have in English. A glossary is a collection of glosses: the terms of one book or field, defined for that use. Every textbook has one at the back; every good contract should. When a word has a special meaning in a field (consideration in law, theory in science), the glossary is the right tool and the dictionary will mislead you.
Which one, then
- You met a word you don’t know: the dictionary. If its definition is as hard as the word, a dictionary that explains instead.
- You have a word and want a better one: the thesaurus, and then the dictionary to check the one you picked, because the thesaurus will not tell you the difference.
- You are inside a field: the glossary, because the dictionary’s meaning may not be the one in use.
- You want to remember the word: none of the three. A dictionary that keeps and reviews your words, or a flashcard app, does that.
And the name of this site: a gloss is the explanation in the margin, written for the reader in front of it. That is the whole idea.
Questions people ask
What is the difference between a dictionary and a thesaurus?
A dictionary gives meanings, pronunciation and origin for a word you look up. A thesaurus gives other words with similar meanings, so you can find a better one for a word you already know. The thesaurus does not usually tell you how the words differ.
What is a glossary?
A list of the special terms used in one book or subject, each with a short definition for that use. It comes from the Greek glossa, tongue; a gloss is a single such explanation.
What is a gloss?
A brief explanation of a difficult word, originally written between the lines or in the margin of a text. A glossary is a collection of glosses. Gloss, the app and this site, is named for it.
Is Google a dictionary?
Typing “define X” into Google shows a dictionary entry licensed from Oxford Languages, so for a quick meaning, yes. It will not keep the word or help you learn it.
Sources
Keep reading
Latin and Greek roots behind thousands of English words · The best dictionary apps for iPhone in 2026 · Words to use instead of “very.” · All guides