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. SLDs are often used to identify specific organizations, businesses, or individuals on the internet. SLDs can be customized to reflect the name or purpose of a website, making them essential for branding and online identity. They are registered through domain registrars and can vary in length and complexity, allowing for a wide range of possibilities for website owners.