Dictionary Lookup in EPUB Readers: How It Works on Each Platform

June 12, 2026  ·  6 min read

One of the advantages of reading on an EPUB reader over a physical book is instant dictionary lookup — tap a word and see its definition without leaving the page. Here's how each major platform implements this feature and how to customize it.

Apple Books

Tap any word → tap Look Up in the popup. The result shows:

• Dictionary definition (from the New Oxford American Dictionary or language-appropriate equivalent)
• Wikipedia summary
• Siri Knowledge (topics, people, places)
• Web search results

Changing the dictionary: Settings → General → Dictionary → select additional languages. Downloaded dictionaries work offline.

Multiple dictionaries: Apple Books can show results from multiple downloaded dictionaries simultaneously. Tap the dictionary name to expand its results.

Offline: Works fully offline once dictionaries are downloaded. The Oxford American, Oxford British English, and many other language dictionaries are available for free download.

Kindle

Tap a word → the Vocabulary Builder popup appears with a brief definition. Tap the definition for the full entry.

Vocabulary Builder: Kindle automatically adds looked-up words to your Vocabulary Builder list (accessible from the main menu). You can review words and mark them as learned.

Changing the dictionary: Settings → Reading Options → Language & Dictionaries → Dictionary. Select from installed dictionaries. Many language dictionaries are free from the Kindle Store.

X-Ray: Kindle's X-Ray feature (available for eligible books) shows all mentions of characters and topics across the book — related to dictionary lookup but more book-specific.

Offline: Kindle's built-in dictionaries work offline once downloaded.

Kobo

Tap and hold a word → tap More → tap Dictionary. Kobo uses the built-in Oxford dictionary for English.

Kobo Plus dictionaries: Language dictionaries are available in the Kobo Store. Some are free, others are paid add-ons.

Wikipedia integration: Tap Wikipedia in the lookup panel for a broader search. Requires internet connection.

Offline: The Oxford English dictionary is built-in and works offline. Additional language dictionaries require download.

Translation: Kobo supports quick translation via a translate option in the word popup. Powered by Bing Translate — requires internet.

Thorium Reader

Select a word or phrase → the reading toolbar appears with a search icon. Thorium does not have a built-in dictionary; instead it offers:

• A search bar that searches within the current book
• External search: selected text can be opened in a web browser search

For serious dictionary lookup workflows with Thorium, the best option is to use the OS-level dictionary:

• macOS: Three-finger tap on a trackpad (or Ctrl+Cmd+D) opens the macOS Dictionary app for selected text.
• Windows: No built-in equivalent; use a browser extension or the Windows Dictionary app.

Moon+ Reader (Android)

Tap and hold a word → tap Dictionary in the popup. Moon+ supports multiple dictionary sources:

• Built-in: WordNet-based English dictionary included.
• ColorDict: Moon+ integrates with ColorDict, a dictionary app for Android that supports offline Stardict-format dictionaries (thousands of free language pairs available).
• Online: Configurable web lookups (Google Translate, Merriam-Webster, others).

Custom dictionaries: Copy any Stardict dictionary (.dict.dz + .idx + .ifo files) to the Moon+ dictionary folder. Moon+ Pro scans and indexes them automatically.

Offline dictionaries for language learners

For reading in a foreign language, offline dictionaries are essential. Options by platform:

FAQ

Does dictionary lookup work in converted PDF-to-EPUB files?

Yes, as long as the EPUB contains real selectable text. EPUBs produced by toolkit.bot have proper text — dictionary lookup works in all the apps above. PDFs converted with image-only output (no OCR) would not support lookup.

Can I use a bilingual dictionary (e.g., English-Japanese) for foreign language reading?

Yes. Kindle has bilingual dictionaries available in the Kindle Store. Moon+ Reader supports Stardict bilingual dictionaries. Apple Books and Kobo support bilingual dictionaries for major language pairs via their respective stores.

What is the best EPUB reader for foreign language reading?

Kindle for its Vocabulary Builder and wide dictionary selection. Kobo for built-in translation. Moon+ Pro for Android users who want Stardict bilingual dictionaries. Apple Books for iOS/Mac users reading in languages supported by Apple's dictionary collection.