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?