The Merriam-Webster Dictionary app for Office lets you search for definitions from within a Microsoft Word document.
Simply select a word in a document you are writing or reading, right-click, and choose Define from the context menu. Definitions display in a handy pane to the right of the document. Click on the link below the definition to view the full dictionary entry on Merriam-Webster.com.
Bonus feature: Helpful definitions from the Merriam-Webster Dictionary will also enhance familiar Microsoft Word tools that you use every day, like the Spelling and Grammar checker and the Thesaurus.