Look up words and phrases while reading an EPUB using an offline StarDict dictionary stored on the SD card.

The reader supports StarDict dictionaries. When searching for dictionaries online, look for "StarDict format" or files with .dict, .idx, and .ifo extensions.

A dictionary folder must contain:

• .idx — word index (required, must be uncompressed — a .idx.gz will not work; decompress it on your computer with gzip -d first)
• .dict or .dict.dz — definition data (.dict.dz is supported as-is; entries are decompressed on the fly during lookup)
• .syn — synonym index (optional; maps alternate spellings and irregular forms to their headword)
• .ifo — metadata (optional)

Not supported: dictionaries with 64-bit index offsets (idxoffsetbits=64 in the .ifo — rare, and rejected with an error).

1. Copy your dictionary folder(s) to /dictionaries/ on the SD card — one dictionary per folder, e.g. /dictionaries/webster/webster.idx + webster.dict.dz. A hidden /.dictionaries/ folder (dot-prefixed) works the same way, for keeping it out of the file browser.
2. Open Settings → Reader → Dictionary on the device.
3. Select a dictionary from the list, or None to disable lookups.

The Dictionary setting only appears when at least one usable dictionary folder exists. Folders containing more than one dictionary (multiple .idx stems) are skipped as ambiguous.

Two ways to start a lookup while reading:

• Open the reader menu (Confirm) and choose Look Up.
• Or set Settings → Controls → Long-press Menu to "Dictionary", then hold Confirm (~0.4s) on the reading page.

On button-driven devices:

1. A selection cursor appears on a word. Use Left/Right to navigate between words in reading order, and the side Up/Down buttons to jump between lines or flip across nearby pages.
2. Press Confirm (Select) to anchor the start of the selection.
3. Single word: Press Confirm again immediately (Look Up) to look up the highlighted word.
4. Phrase / multiple words: Use Left/Right or Up/Down to extend the highlighted selection across words, lines, or pages. Then press Confirm (Look Up).
5. Press Back at any time to cancel and return to reading.

On touchscreen devices:

• Tap or drag the selection handles across words to highlight a word or phrase, then tap Look Up.

On the very first lookup with a dictionary (and again whenever the .idx or .syn source file changes), the reader shows "Indexing dictionary…" while it builds small sidecar files next to them — a .qidx for the word index, and a .sidx when a .syn synonym file is present. Each sidecar is rebuilt independently, only when its own source changes. This takes a few seconds for large dictionaries and makes all subsequent lookups fast. The sidecars can be deleted safely at any time — they will simply be rebuilt.

1. Direct match — the word or phrase is searched as-is (case-insensitive) in the dictionary index. Leading and trailing punctuation and whitespace are stripped automatically. Multi-word phrases match dictionary entries containing spaces (such as idioms, phrasal verbs, or compound terms).
2. Synonyms — on a miss, if the dictionary ships a .syn file, alternate spellings and irregular forms recorded there are resolved to their headword (e.g. oxen → ox, colour → color). This step is skipped if the .sidx sidecar could not be built (e.g. transient low memory during indexing); the dictionary otherwise stays usable, and the build is retried the next time it is opened.
3. Stemming — still no match: common English word forms are retried automatically: possessives and plurals (dogs → dog, stories → story) and verb endings (walked → walk, running → run, making → make).
4. Not found — a short popup appears and you return to word selection.

When a word is found, the definition screen shows the matched headword at the top and the definition text below, with a page counter for long definitions.

HTML dictionaries (sametypesequence=h) and XDXF dictionaries (sametypesequence=x) use the EPUB text-layout engine for supported formatting such as headings, bold, italics, lists, and line breaks. Unrecognized tags keep their text without special formatting. Images and CSS styling are ignored. Definitions that are too large or cannot be parsed or laid out within the available memory fall back to plain text.

• Left/Right or side Up/Down — previous / next page
• Back — return to word selection

credit to https://github.com/koreader/koreader/wiki/Dictionary-support for the list.

• The reader.dict (ex "BoboTiG/ebook-reader-dict") project provides StarDict version of daily dumps of Wiktionary monolingual dictionaries for a variety of languages. It also provides non-free multilingual dictionaries.
• The WikDict project provides free bilingual dictionaries based on Wiktionary for a lot of language pairs. StarDict versions can be downloaded from here.
• The Vuizur/Wiktionary-Dictionaries repository contains dictionaries based on Wiktionary from many languages to English, including English-English.
• The DictInfo website provides outdated monolingual dictionaries based on Wiktionary.
• The Firedict site contains a list of freely available dictionaries.
• wiktionary_stardict: update monthly.
• Fictionaries provides dictionaries for various speculative fiction books and series.
• World Factbooks Archive provides 36 years of CIA's World Factbook dictionaries in StarDict format.
• StarDict-Hebrew Hebrew-English StarDict versions of Babylon dictionaries.