If you are looking for English-to-English dictionary, which is free and can be used offline (without requiring Internet Access). Then Artha from http://www.artha.sourceforge.net/ is a good choice. Artha is a free English-to-English dictionary and thesaurus for Ubuntu/Linux based on WordNet. It completely works off-line.
To install Artha, open terminal and type:
sudo apt-get install artha
$ sudo artha
or use dashboard search: