The Insights feature will hook up with Bing to look for the relevant information, where it will also perform a search on Wikipedia as well as the Oxford English Dictionary, where the latter would provide definition for words.
Microsoft mentioned, “When you right-click on a word and select Insights, Office Online understands what you are looking for and then uses the power of Bing to bring in relevant information from a variety of sources like Bing Snapshot, Wikipedia, Bing Image Search, Oxford English Dictionary and the web—all neatly presented alongside your content. You can spend as much time as you want in the Insights experience—take a quick glance or explore details and interesting articles related to your selection, and do all this while you are working in Word Online.” [Press Release]