The domain name itself or being technically accurate - the Second Level Domain (user-defined) has very few requirements. A common requirement is about the length of a Domain Name - it can be between 4 and 63 characters long.
Depending on the Top Level Domain, also known as Domain Extension, the requirements can vary, and everyone needs to comply with those before registering a Domain Name. We strongly suggest for our customers to check the dedicated page for the Domain Extension that they are about to register so they can be fully aware of the requirements.