A family of Microsoft word processing software products for creating web, email, and print documents.
On the Home ribbon, click the Add-ins button. At the bottom of the dropdown that opens, click the More Add-ins button. Just below the title of the Office Add-ins dialog, click the STORE item if it isn't already selected.
At the top of the right column of the dialog, click the Sort By item and choose Name. In the left column, click the Reference item. Scroll the list to find "English Dictionary". (Note: do not choose "Dictionary - Merriam-Webster"; that add-in is broken.) Click the Add button next to that add-in, and click the Continue button in the prompt that follows it.
Alternatively, you can search the internet for other dictionaries that interact with Word. I don't have any specific ones to recommend.