Use a Domain You Already Own (Domain Mapping)
Do you already own a domain name? If you would like to use it here on WordPress.com all you need to do is “map” your existing domain to your site. You will need an upgraded WordPress.com plan on your site in order to map a domain. Each paid plan comes with a free domain credit you can use to either register a new domain or map one you already own. You can map as many additional domains to the same site as you would like, but you have to pay for additional mapping upgrades separately.
If you’re trying to map a subdomain (for example: blog.yourgroovydomain.com instead of yourgroovydomain.com), use our Map A Subdomain support page instead.
Read in WordPress Support Blog