desktop dictionary

[Peter G.]

Is there a dictionary that can be used with okular, 
firefox, other programs, such that:

when reading a file in any of the aforementioned 
programs, one need only click on a word and the meanings 
of any English, German, French and other language words 
will appear (the definitions for these languages need not 
be translated, but definitions for other languages, if 
supported, must appear in one of these languages).