A second-level domain (SLD) is the section of a domain name that is to the left of the dot, while a top-level domain (TLD) is the section to the right of the dot, also known as the domain extension. For example, our domain name is dynadot.com, with "dynadot" being our SLD and ".com" being our TLD. This part of the domain is known as "second-level" due to the hierarchy of the Domain Name System (DNS).

Sometimes an SLD can be considered part of the domain extension as some domain registries use an SLD to indicate the usage of the TLD. For example, the .UK registry offers .CO.UK for general/commercial use, .ORG.UK for non-profits, and .ME.UK for personal websites. These three examples are known as ccSLDs or country code second-level domains.

Also please note that some registries do not allow direct registrations on the TLD. This was the case for .UK until direct registrations were opened in June of 2014.

What is a third-level domain?
What is a subdomain?