alias dict 'curl dict://dict.org/d:\!*'Now to look up a word, just type dict word_to_define.
[Editor's note: There's at least one service (OmniDictionary) which will return the definition of a selected word if you prefer the GUI for your dictionary browsing ... but this is quite nice when you don't want to touch the mouse or the menu to define a word. Note that I have substantially (completely?) rewritten what Carthag submitted ... primarily because, although it was fun reading an IRC transcript, it really didn't fit the layout of the macosxhints page all that well! ;-)]

