When you register a domain, a vanity domain is characterized by the fact that the essential part of one of your URL subdomains is not a complex, code or set of numbers, but rather an individually chosen name or term which represents the owner. A good vanity URL is short, meaningful, and avoids using unnecessary words or numbers, making it easy to remember while still appearing trustworthy.
The main component of the URL could be, for example, the name of a person, company, brand, product or a made-up name. Incidentally, a uniquely named social media profile is also referred to as a vanity domain — regardless of whether a content-optimized vanity URL exists.