Pick a Dictionary to give you a definition in Word 2013
1. RIGHT click a word in your Word 2013 document
2. Select Define (with a left click)
3. If this is the first time you have used this a side panel will open giving you a choice of dictionaries to download.
4. Choose a Dictionary (most are Free) and click the Download button - then wait
5. Now with the dictionary side panel open all you need to do is double click on any word and you will see a dictionary definition appear in the right panel
Tip - if the Dictionary side panel is closed - just carry out steps 1 and 2 again