This includes the SLD, and TLD of a web address, or email address. In this case, fsp.fyi is the domain name. For those of you who uses Facebook, then they will go to the domain of facebook.com. While not required, you can type domains in capitals, and the domain is not case sensitive. Therefore, FSP.fyi is the same as fsp.fyi.
One thing to note is you might see me making a domain reference ending in .tld. There are no .tld domains, and I don’t see that ever happening. I do this with any domain I don’t own, and don’t want to provide an unwanted attention by accidentally using someone else’s domain. If .tld domains were to become available, then I would adjust my pages to reflect that as soon as possible.