A Second-Level Domain (SLD) is the part of a domain name that comes directly before the top-level domain (TLD). For example, in the domain name example.com, "example" is the SLD, while ".com" is the TLD. The SLD often represents the name of a business, organization, or individual, making it a crucial part of a website's identity. SLDs are used in various domain extensions, such as org for organizations or net for network services. They help users easily identify and remember web addresses, contributing to effective online branding and navigation.