Imagine that you want to have fast, offline access to a dictionary. To multiple dictionaries, glossaries, and encyclopedias. Perhaps you want to store this data in a way that can be used across multiple systems, without relying on a local dictionary server (like dictd or dicod). Perhaps you would like the option of using either a simple, scriptable command to query those dictionaries, or a full GUI.
You need StarDict. The primary project is at stardict-4.sourceforge.net.
User dictionaries are stored in ~/.stardict/dic; system dictionaries in /usr/share/stardict/dic.
As an example, the following files exist in ~/.stardict/dic/stardict-wordnet-3.0.0/:
Dictionaries are gzip compressed (the dz file).
A good source for dictionaries in StarDict format: http://download.huzheng.org/
The GoldenDict site has a list of dictionary sources.
Wikipedia and Wiktionary
There are some Wiktionary dictionaries in Stardict format available from dictinfo.com. These are getting old though; as of this writing most of the files there are from the year 2018.
Regarding Wikipedia database conversion, the following links may be interesting:
Converting a dictionary to StarDict format
The KTranslator project provides dictconv, a tool for converting between dictionary formats. It converts Babylon, Freedict, and Sdictionary formats to StarDict. In Debian, just install dictconv.
Stardicter is a Python tool for converting various dictionary formats to the StarDict format.
The stardict-4 project provides utilities for converting DICT, wquick, mova, and pydict formats to StarDict. In Debian, just install the stardict-tools package.