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Dictionary

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

Supported Format

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).

Setting Up a Dictionary

  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.

Looking Up a Word

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.

One word on the page becomes highlighted:

  1. Use Left/Right to move between words in reading order, and the side Up/Down buttons to jump between lines.
  2. Press Confirm to look up the highlighted word.
  3. Press Back to return to the reader.

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.

How Lookup Works

  1. Direct match — the word is found as-is (case-insensitive) in the dictionary index. Surrounding punctuation is ignored.
  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.

The Definition Screen

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 that declare sametypesequence=h use the EPUB text-layout engine for semantic formatting such as headings, bold, italics, lists, and line breaks. Images and CSS styling are ignored. Definitions that are too large or cannot be 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

Where to find dictionaries

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