Here's what the Mac OS X help says about this:Mac OS X includes Dictionary, an application you can use to look up definitions of words in a dictionary, as well as synonyms and antonyms in a thesaurus. You don’t need to be connected to the Internet to look up words using Dictionary. To quickly look up words while working in applications such as Mail or Safari, select a word, Control-click it, and choose “Look Up in Dictionary.” The definition appears in the Dictionary window or in a small panel next to the selected word, depending on your Dictionary preferences.
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