As regards having an offline encyclopedia, there are some programs that provide wikipedia offline. They typically download a large compressed snapshot of wikipedia to your harddrive, to access offline. I once installed
kiwix, and it worked okay. But other offline wikipedia programs have been mentioned on the
ghacks site too.
For an offline dictionary, you might consider gnome-dictionary, opendict, or xfce4-dict - the all provide similar features, but differ slightly in their presentations. For offline access, they also need
dictd (a dictionary server) installed. The nice thing about
dictd is that some standard dictd dictionaries (such as dict-wn, dict-foldoc, dict-gcide, dict-jargon, and dict-moby-thesaurus) are avail to install from most repos. Much easier than having to fosick about the net looking for dictionary files of the right format! Elsewise, the
dictd dictionary server runs in the background, and gnome-dictionary etc. are clients that forward their queries to it. These howtos may be helpful...
http://ubuntuforums.org/showthread.php?t=145949
http://linuxers.org/howto/how-make-g...y-work-offline
The other main Linux dictionary program to consider is stardict, which I haven't really used. It is talked about more
here.