One of the less-known features of Mac OS X in the Dictionary application. In the Dictionary app, you can type a word to look it up in the New Oxford American Dictionary, Oxford American Writer's Thesaurus, Wikipedia and Apple Dictionary.
You'll find Dictionary in your Applications folder. Launch it, and select the source you want to search. For example, to search just the dictionary, click "Dictionary". Click "All" to search all sources, including Wikipedia. Type a word into the search box. Hit return and you'll see the results.
There are other ways to quickly access Dictionary.
Spotlight helps you quickly find things on your computer. Spotlight is located at the top right of the screen. Type a word or phrase into Spotlight. In the list of results, you will see a "Definition" section (assuming the word is in the Dictionary). Select the Definition and Dictionary will launch, showing you matches in the Oxford dictionary.
In some applications, like Bare Bones BBEdit, TextEdit and Apple Pages, you can select a word, then right-click it (or control-click). Select "Look it up in Dictionary" from the pop-up menu to search for the word in Dictionary.
You can also place the mouse pointer over a word, and press control-command-D to bring-up the Dictionary panel. In the Dictionary panel, you can choose to view Thesaurus results.
And, you can click "More..." to open Dictionary to see the complete definition.
Mac OS X and the Dictionary application make it easy to find definitions, synonyms and Wikipedia entries.