title_body: Are dictionaries supported in epub 3.1?
upvoted_answer: Are dictionaries, as described here [...]
  
  
  The dictionaries and glossaries specification is not a part of the epub3.x standards. 
  
  
    Which readers support them? 
  
  
  None that I know of. 
  
  
    Are there any working examples of epub documents using dictionaries?
  
  
  No. 
  
  If you want to use dictionaries, you'll have to create StarDict dicionaries, which are supported by most Android devices, but not by iOS devices. (There are iOS StarDict dictionary apps, but you can't use them as lookup dictionaries in iOS ebook apps because of iOS limitations.)
  
  If the dictionary is rather small, you could theoretically embed it as an Sqlite database in epub3 books and query it with JavaScript.