There are a few ways of using built-in Dictionary in Mac OS X

You can simply open the Dictionary from Applications folder and key in your search criteria:

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When in Cocoa applications, such as Safari or Mail (but not Firefox, Camino, Thunderbird), you can hold Ctrl+Cmnd+D on your keyboard and hover your mouse pointer above the word, and the Dictionary definition for that word will pop-up.

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You can also select and right-click the word and select Look Up in Dictionary from the contextual menu, this will open the Dictionary with the definition displayed.

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And if your Mac is running OS 10.5 Leopard, you can also search the Dictionary definition through Spotlight. Just open the Spotlight and type your word in, one of the result lines is the (Dictionary) Definition.

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You can either click on it to open the Dictionary, or just hover your mouse to read the definition as a tool tip.

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