Try Google Docs Dictionary and Thesaurus

Google Operating System had already uncovered that Google is working on integrating Encyclopedia Britannica and Meriam-Webster’s dictionary to Google Docs. This feature is yet to be official, but thanks to some extra ordinary research by Tony Ruscoe, you’ll be able to start using the dictionary with the help of a short javascript line. Here’s how:

1. Open a Google Docs document
   
2. Paste the following code into your browser’s address bar and hit return:
   javascript:void(INCLUDE_BRITANNICA_INTEGRATION=true);
   
3. Right-click any word in your document so that you can view the extra items that have been added to your menu through the use of the JavaScript code.
   

Once done, you’ll notice three new options added whenever you right click on any word. Although Encyclopedia articles doesn’t work, the other two (dictionary definition and thesaurus tool) work pretty decently.

You’ll have to paste the javascript line every time you edit a document on Google Docs, which is quite annoying until this feature becomes officially available. So to avoid this problem, Haochi from Googlified has created a greasemonkey script which puts this hidden feature in always on mode.