Both major desktop environments have dictionary GUIs that usually connect to a website such as dict.org and other on-line dictionary sites to look for definitions. KDE has KDict and GNOME has Gdict, both of which may be installed from your distro's mirrors. Gdict is in the package "gnome-utils".
If you are using another environment or window manager, wordinspect is your app. It is rare <a href="http://newbieslinux.com/glanz/upload/18054wordinspect_0.1a-8.2_i386.deb">so I will mirror it here</a> for deb users.
The trick is to go into your dictionary GUI front-end's preferences, and select or set the server from the on-line one to localhost after you have installed the dict, the dictd packages and the dictionaries.
You will need the following in addition to the front-ends mentioned above: dict dictd You'll need dictionaries, of course. I recommend the following: dict-gcide This package contains the GNU version of the Collaborative International Dictionary of English, formatted for use by the dictionary server in the dictd package. The GCIDE contains the full text of the 1913 Webster's Unabridged Dictionary, supplemented by many definitions from WordNet, the Century Dictionary, 1906, and many additional definitions contributed by volunteers.
Don't worry about this being out of date. It is up to date because of contributors and remains refreshingly politically incorrect.
dict-jargon Jargon File 4.4.4 This package contains The On-Line Hacker Jargon File, version 4.4.4, 14 Aug 2003, formatted for use by the dictionary server in the dictd package.
dict-moby-thesaurus Largest and most comprehensive thesaurus Moby Thesaurus is the largest and most comprehensive thesaurus data source in English available for commercial use. This second edition has been thoroughly revised adding more than 5,000 root words (to total more than 30,000) with an additional _million_ synonyms and related terms (to total more than 2.5 _million_ synonyms and related terms).
dict-vera V.E.R.A. - A Dictionary of Computer Related Acronyms This package contains V.E.R.A. -- Virtual Entity of Relevant Acronyms, formatted for use by the dictionary server in the dictd package. This is based on gnu version 1.15 of V.E.R.A. (upstream version 6.9)
dict-foldoc FOLDOC Dictionary Database This package contains the FOLDOC, the Free On Line Dictionary of Computing, supplemented by a number of Debian-specific definitions, formatted for use by the dictionary server in the dictd package.