I had done a lot of googling in the past to find out a dictionary that I could use in Ubuntu when I'm not connected to the internet. It yielded nothing, atleast not anything that is ready-packaged for Ubuntu.. Few days ago I was just going through the 'Ubuntu Software center' to find out any random software that may interest me. Well, I found something that interested me; 'Artha'.
Artha is a pretty decent dictionary which works with Wordnet 3.0 as its backend which is very comprehensive(atleast for common use, if not more). It has many features like 'relatives' of the word, which includes "Synonyms, Antonyms, Derivatives, Similar, Domains, Causes, Kinds(Hyponyms)". In my experience I found it very good and indeed comprehensive.
The best feature that I liked in it is its HotKey which is Ctrl+Alt+w. This is a very handy feature, you can just select a word in any software and press the Hotkey combination to invoke Artha, its notification/window will then come up with the meaning, as per your settings selection.(I like the Notifiaction feature)
The version of Artha found in Ubuntu 9.10(Karmic) repository is 0.8.0. But, Artha is available in version 1.0.1 for Ubuntu and many other Linux distributions. It has regular expression based search.You can find the download details for the new version for Ubuntu and other Linux distributions, at the following link:
Download Artha 1.0.1