If you spend a lot of time at the shell, you might prefer to look up the meaning of words from the commandline, rather than from a GUI program or in the browser. This can be done using the Console Version of StarDict program.
The Console Version of StarDict program can be installed using the package name sdcv. No dictionaries or resources are installed along with it.
You can find dictionaries that are English-English or between any two languages, encyclopedias, thesaurus or etymology sources online that can be installed to work with sdcv. For example, a few of such dictionaries can be found here.
To install a dictionary, first create the /usr/share/stardict/dic directory, if it does not exist. After that, unzip the dictionary file and copy that directory to /usr/share/stardict/dic You will need superuser privileges to do this.
You can list the dictionaries that are installed using:
$ sdcv --list-dicts
To look up a word, say mollify:
$ sdcv mollify
Tried with: Console Version of StarDict 0.4.2 and Ubuntu 12.04 LTS