Mac OS X has had for ages a feature that's been keeping me from trying or moving to other platforms: its built in Dictionary.app with American & British dictionaries as well as thesauri, plus dictionaries of various other languages. The fact that it's possible to triple-tap (or double-tap + "Look Up ") on a phrase of text/word in virtually all applications to have a dictionary pane pop-up, or search for a word in Spotlight and press enter for a full explanation of that word, is a major advantage.
Now, all this OS X praise aside: is it possible to achieve something equivalent/comparable in Ubuntu or any of its flavors?
Best Answer
Check out GoldenDict. It's one of my favorite pieces of software. Cross-platform compatible dictionary look-up! It's highly customizable and compatible with a plethora of different dictionary formats and pretty much any website out there (e.g. wikipedia, dict.cc, etc.):
You can either install it from the USC or get one of the latest development releases from the OpenSUSE build repository (e.g. for 12.04 LTS/amd64).
As far as dictionaries are concerned, I'd suggest you install Wordnet 3.0 to get started. Some notes on supported dictionary formats and where to find them may be found here.