OS X Tip: Quick Word Definition (Dictionary) Pop-up
One of my favorite features in Mac OS X is the ability to quickly retrieve the dictionary definition of a word from virtually any application.
Move your mouse cursor over a word from any Cocoa application (Safari, TextEdit, etc.), then hold Ctrl and Command (⌘) and tap D. This will cause a small pop-up window to appear with the definition of that word.
This is great when reading things online from authors that like to use big words.
I often use the dictionary/thesaurus widget on the dashboard. I have to type in or copy/paste the word but can use it anywhere (as long as I’m online).