There are more than 350 million domains registered and any one of them might be your premium domain name. However, in reality, only a few of them are considered as real premiums and they have somethings in common. Let's get into the details.
First of all, the extension of the domain name is very crucial considering there are thousands of extensions out there. .COM is the most commercial and well-known extension right now and we mostly look for the .COM extension in the premium domain market.
The keyword or phrase is very important as well. It has to be a category-defining such as "wedding dress" and/or generic, such as "wedding" term to be considered as a premium.
Moreover, the length of the domain is a key factor. Public researches show that the users are having a hard time remembering long names.
Language is another factor here. English language and therefore mostly English keyword domains considered premium world-wide. There're lots of premium domains in other languages but they're mostly premium for one country or for a few countries. For instance: Domain with a French keyword might be premium for the countries that use French in their daily life.
There are lots of more criteria for the domains to be considered as premiums but these are the most common ones.