What Does Second-Level Domain (SLD) Mean?

A second-level domain is a specific part of a website, page domain name or URL address that complements a top-level domain. One of the easiest ways to define a second-level domain is that it consists of that portion of the domain name to the left of the “.com” or other similar extension, which is called a top-level domain. Analysis of the top-level and second-level domains helps to analyze a URL or page address.

Techopedia Explains Second-Level Domain (SLD)

In a very general sense, a second-level domain is often thought of as the “name” of the domain. The top-level domain, which is an extension such as “.com,” is fairly generic. Although it is the controlling address feature, it does not help to distinguish a site from others. The second-level domain often plays this role; for example, in a domain name like “google.com,” the word “google,” as the second-level domain, is where domain holders put the brand name, project name, organization name or other familiar identifier for users.