As a teacher, I find that gnome-dictionary and its applet are useful tools. But at first it was much too slow and required Internet access. Here are steps to install a database of definitions on your system so that gnome-dictionary works much faster and without Internet access.
1. Install the packages dictd, dict-gcide, and dict-wn.
-- dictd is the dictionary database "server" of sorts
-- dict-gcide is a comprehensive free English dictionary
-- dict-wn is another dictionary with more up-to-date definitions, but it is not comprehensive
2. Install other dictionary packages you might want
-- Look in Synaptic at the many packages under dict- and read their descriptions. If you want to download all the dictionaries, you can just get the metapackage dict-freedict. I avoided this option because I did not want to clutter my definition screen or to wait while the program searched all these extra dictionaries I'd rarely (if ever) use.
-- dict-moby-thesaurus is available if you want a thesaurus, too.
3. Change the preferences in gnome-dictionary.
-- Applications > Dictionary opens gnome-dictionary
-- Edit > Preferences
-- change server to localhost
-- change the Database to "search all databases"
4. work in progress: Change the order in which definitions appear
-- The file /etc/dictd/dictd.order apparently controls the order in which definitions are searched, the order in which they appear, or maybe both. I want the up-to-date definition (from wn) to be listed first when they are available, so I changed my order to "wn gcide web1913..." It does not appear to have made any difference -- the old-fashioned definition still appears first.