MS Word 2013 No longer includes a Dictionary feature
When you click on the Define option under the Review Tab, a message opens on the right side to Sign In with a Microsoft account login.
An Add a Service window will open to enter your email or phone# for the sign in from your own personal Microsoft account or Microsoft Online Services ID to access it.
WORKAROUND IF YOU DO NOT HAVE A PERSONAL MS ACCOUNT: Hold the ALT key while clicking a word in the text. A list of online dictionaries will display on a right hand side panel that you can choose to define the word. Notice that from the drop down arrow you may select Encarta Dictionary.
Word 2013 has two separate ways to look up word meanings.
1. The Define button on the Review tab does require a dictionary from the Apps for Office to be installed.
2. The other mechanism, (the work around above) which remains from previous versions.