Word 2013 has a built in Merrium-Webster dictionary. Right click a word you want to look up and click choose “define”. Or click on the Review tab and then click “define”.
Using Word 2007 or 2010, right click the word then select “research” to open up the research task pane. From the task pane select the dictionary option.
Also you can press the ALT key and left click a word to have the research task pane show up using Word 2007, 2010 and 2013. The research task pane has options such as Bing, translation tools, thesaurus, and more.