A defining vocabulary is a list of words used to define other words. It is a very disciplined glossary where words are defined only in terms of simpler words, and where they are typically only defined in their most common usage. In projects that have a defining vocabulary, it is an error to use any words that is not in that vocabulary, to explain any other word, in it, or not.
An w:idiom dictionary or w:translation dictionary must, necessarily, rely on a defining vocabulary, to ensure that words are not used in confusing or idiomatic senses that a native speaker of that language won't understand.
An example of a good professional project that has such a defining vocabulary is the Longman's American Idioms dictionary which defines the 4000 most common idioms in American English, and effectively explains a great deal of American culture in one small book, all using 2000 words only in their simplest senses with the simplest prefixes and suffixes. This is a good basis for explaining any glossary item that might be factionally defined or culturally defined. With less than 2000 words, it is impossible to deal with these differences or explain these cross-cultural concepts to another culture, or at least, there are no examples of it being done successfully.
Very unwisely, the Simple English Wikipedia has refused to accept the 2000 word threshold, and also refused any strategy of staging entries so that their first paragraphs are easily readable (in say 1000 words) but their second and subsequent paragraphs can actually explain complex concepts with reasonable certainty of being understood. This one failure has removed it from our list of essential projects, and it shows the importance of a proper defining vocabulary, and not going through a futile attempt to do with amateurs using 1000 words, what Longman's barely manages to do in 2000 words with professionals. The failure of the SEW to actually serve as a neutral basis for translation will probably be the first obvious failure among enemy projects. By no means should their policies be emulated or adopted here!