How do I re-install Word on my Mac?

You will need to repeat the process by which it was installed the first time. Unless the icon is simply missing from your Dock or Desktop. If that's the case, try looking for it in the "Applications" folder. If it truly was erased from your Mac you'll need to determine how it was installed originally. From a disk? From the internet? Subscription or bought? Yopu may need proof-of-purchase like a license key.

Without an authorization key or install disk you will have to purchase one from Microsoft.

Use the install media you got when you purchased Microsoft Office or re-download it from Microsoft.

Good to know: it’s possible to use Pages to open documents that were made in Word. You can also create new documents in Pages and save them as a Word-doc

How was it installed to begin with?