QStarDict is a free (licensed under GNU GPL version 2) dictionary application for GNU/Linux.
It is developed to work on X11 and show a popup window for words when they are selected in any application. The format of dictionaries that is supported is StarDict.
The main window:
The popup window:
StarDict dictionaries are available from repositories of Fedora and Debian.
The source code of QStarDict is available at http://gitlab.com/a-rodin/qstardict.