A subdomain is sometimes termed a vanity domain, especially when it is a subdomain of an ISP's own domain aliased to an individual user account. Other definitions include:
msgVanityDomainto set up a domain name for e-mail use which is not fully hosted.
These uses apply primarily to regular domain names registered at the highest allowed level rather than subdomains (although, technically speaking, a second-level domain is actually a subdomain of its top-level domain).
Because vanity domains are operated for or on behalf of individuals, they typically will not offer the full complement of services an organisational domain name (or rather, the host that it points to) would be expected to honour; for example, the DNS record may contain only an MX record identifying a mail server accepting e-mail for that domain (which may itself be an e-mail forwarding server) and an A record identifying a shared web hosting service only offering  (which may itself be a URL redirection service).
As well as being easily remembered, vanity domains (especially when registered at the highest level allowed by the registrar) offer the advantage of personal mobility; they continue to be associated with a person even when that person switches service provider.