One of the best features of osx is the built in dictionary and thesaurus.
you can open the dictionary (applications > utilities > dictionary) and lookup the definition of any word using the built in dictionary, or even the builtin wikipedia browser!
there is, however, a cool way to lookup the definition of a word without opening the dictionary.
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It is not working for Preview. I opened a pdf in Preview, neither CTRL CMD D is not working nor 3 finger double tap :(